These shells can have a coxswain---a person who steers the shell (using a rudder) and urges the rowers on. I have included in parenthesis the symbol used for each subtype along with some dimensions and weights.
Coxed Pair (2+)
Two sweep rowers with a coxswain.
Coxless Pair (2-)
Two sweep rowers without a coxswain.
Coxed Four (4+)
Four sweep rowers with a coxswain.
Straight (or Coxless) Four (4-)
Four sweep rowers without a coxswain. Steering is usually accomplished via a rudder that is attached to a cable that is connected to one of the rower's foot stretchers (this an adjustable bracket to which the rower's feet are secured). The coxless pair has a similar type of rudder setup.
Eight (8+/8o)
Eight sweep rowers with a coxswain. Eights are 60+ ft (~18.5 m) long and weigh about 250 pounds (~114 kg).

Sculling Boats (each rower has two oars)

Only in rare cases do these boats have a coxswain. Steering is generally accomplished by applying more power or pressure to the oar (s) on one side of the shell. The hands overlap (usually left over right in the US) during part of the rowing cycle, or are always left in front of right.
Single (1X)
One rower or sculler. Singles are about 26 ft (8 m) long and less than a foot (0.3 m) wide. Racing singles can weigh as little as 30 pounds (~13.5 kg). There are heavier (~45 to 50 pounds), shorter and wider versions often referred to as recreational singles.
Double (2X)
Two scullers. Most racing doubles can be also used as a pair with a different set of riggers designed for sweep oars. When used as a pair a rudder is usually added. There are also recreational versions of sculling doubles.
Quadruple (4X)
Four scullers. Often referred to as a `quad' and usually has a rudder attached to one of the sculler's foot stretchers as in the straight four. Most quads can also be rigged as a straight four using a different set of riggers.
Octuple (8X)
Eight scullers. This is rarely seen, though is used in the UK, at least, in junior competition where sweep rowing is not allowed.

Weight Classifications

There are basically two weight classes for rowers---heavyweight (HWT) and lightweight (LWT).
Men (M)
For team LWT boats, there is a 72.5 kg (~160 lbs) individual maximum, and the boat must average no more than 70 kg (~155 lbs).
Women (W)
The individual maximum for team LWT boats is 59 kg (~130 lbs), and the boat must average no more than 57 kg (~125 lbs).
In the US, the women have an individual max only; no average. In some regattas in the US (usually head races late in the season) these limits are increased by 5 lbs.

A rowing shell is usually built with a particular weight class of rower in mind. Until just recently the Olympics effectively had only HWT classifications.

What do the terms used in rowing mean?

Equipment terminology

Blades - The wide flat section of the oar at the head of the shaft, also known as the spoon. This term is often used when referring to the entire oar.
Hatchets (a.k.a. big blades or choppers or cleavers) - A relatively new design of oar blades (although the idea has been around for some time). These were introduced by Concept II (Spring 1992) and are what the names indicate---oar blades that have a bigger surface area than the `standard' (Macon) blades and have a hatchet or meat cleaver shape. The hatchets are a bit shorter (by about 7 cm) than the standard blades.
Scull - This term is used when referring to one of the oars used in a sculling shell, the shell itself or to the act of rowing a sculling shell.
Foot Stretcher (or bootstretchers) - An adjustable bracket in a shell to which the rower's feet are secured in some sort of shoe or clog.
Seat - The sliding seat that the rower sits on. The term "seat" also refers to the rowers place in the boat; the convention is to number the seats from bow to stern, i.e. the rower closest to the front of the boat is "1-seat" the next, "2-seat", et c. The 1-seat is also commonly referred to as "bowseat" or just "bow" while the sternmost (rear) seat is referred to as "stroke seat" or just "stroke".
Rigger - (or outrigger)
The device that connects the oarlock to the shell and is bolted to the body of the shell. On sweep boats, riggers are typically alternating from side to the other on adjacent seats, but it is not uncommon to see two adjacent riggers on the same side. This is referred to as "tandem rigging". Variety's include "bucket rigging", "German Rigging" and "Italian Rigging".
Oarlock (or rowlock) - A U-shaped swivel which holds the oar in place. It's mounted at the end of the rigger and rotates around a metal pin. A gate closes across the top to keep the oar in.
Button (or collar) - A plastic or metal fitting tightened on the oar to keep the oar from slipping through the oarlock.
Pitch - The angle between the blade (on the drive when the blade is `squared') and a line perpendicular to the water's surface.
Slide (or track) - The track on which the seat moves.
Gunwale (or gunnel, saxboard) - Top section on the sides of a shell which runs along the sides of the crew section where the rowers are located. The riggers are secured to the gunwale with bolts.
Keel - Technically, the structural member running the length of the boat at the bottom of the hull. Today, some shells are built without this member so the term often refers to the center line of the shell.
Rudder - Steering device at the stern. The rudder in turn is connected to some cables (tiller ropes) that the coxswain can use to steer the shell. Older shells have short wooden handles (knockers) on the tiller ropes. These knockers are used by the coxswain not only to steer the shell, but also to rap out the cadence of the stroke rate on the gunwale.
Skeg (or Fin) - A small fin located along the stern section of the hull. This helps to stabilize the shell in holding a true course when rowing. All racing shells have a skeg. The skeg should not be confused with the rudder.
Rigging - The adjustment and alteration of accessories (riggers, foot-stretchers, oar, etc.) in and on the shell. Examples of rigging adjustments that can be made are the height of the rigger, location of the foot-stretchers, location and height of the oarlocks, location of the button (or collar) on the oar and the pitch of the blade of the oar.
Slings (or boat slings, or trestles) - Collapsible/portable frames with straps upon which a shell can be placed temporarily.

Rowing cycle terms

Starting with the rower at `rest' and legs fully extended with the oar blades immersed in the water perpendicular to the water's surface.

Release
A sharp downward (and away) motion of the hand which serves to remove the oar blade from the water and start the rowing cycle.
Feathering
The act of turning the oar blade from a position perpendicular to the surface of the water to a position parallel to the water. This is done in conjunction with the release.
Recovery
Part of the rowing cycle from the release up to and including where the oar blade enters the water.
Squaring
A gradual rolling of the oar blade from a position parallel to the water to a position (almost) perpendicular to the surface of the water. This is accomplished during the recovery portion of the rowing cycle and is done in preparation for the catch.
Catch
The point of the rowing cycle at which the blade enters the water at the end of the recovery and is accomplished by an upward motion of the arms and hands only. The blade of the oar must be fully squared at the catch.
Drive
That part of the rowing cycle when the rower applies power to the oar. This is a more (or less) blended sequence of applying power primarily with a leg drive, then the back and finally the arms.
Finish
The last part of the drive before the release where the power is mainly coming from the back and arms.
Layback
The amount of backward lean of the rower's body at the end of the finish. Now we start again with the release and ...

Other terms of interest
Bow
The forward end of the shell. Also used as the name of the person sitting nearest to the bow.
Stern
The rear end of the shell.
Port
The left side of the boat when facing the bow (stroke side in the UK and Ireland).
Starboard
The right side of the shell when facing the bow (bow side in the UK and Ireland).
Coxswain
The person who steers the shell and urges the rowers on during practices and in a race. A knowledgeable coxswain can also serve as a coach for the rowers and can be the difference between winning and losing a race.
The Stroke
The rower sitting nearest the stern (and the coxswain, if there is one). The stroke is responsible for setting the stroke length and cadence (with the coxswain's gentle advice).
Frig rigging
See Tandem Rigging.
Tandem rigging
Variations of rigging of sweep boats with adjacent riggers being on the same side of the boat. Also known as Frig rigging (UK). See below (the rigging terms below are the subject of debate as to exactly what configuration they refer to, and they are often used interchangeably).
Bucket rigging
The rigging of an eight or a four so that riggers 2 and 3 are on the same side.
German rigging
The rigging of an eight so that riggers 4 and 5 are on the same side while the others alternate.
Italian rigging
The rigging of an eight so that bow and stroke riggers are on the same side, with the others alternating in pairs.
Ratio
The ratio of the recovery time to the drive time. The recovery time should always be longer than the drive time (how much longer I won't say ... as someone wrote, the idea is to `move the boat on the pull through (or drive) and take a ride (i.e. relax) on the recovery without sacrificing the very speed that they have generated').
Rating
The number of strokes per minute. Also known as stroke rating.
Set (set of a boat)
The definition that I think comes closest to what rowers mean by the set of a boat is `form or carriage of the body or of its parts'. In this case the `body' consists of the shell and the rowers. Items that can affect the set of the boat are the rower's posture, hand levels, rigging (the favorite culprit ... especially with the more advanced rowers), timing at the catch and release, and outside conditions such as the wind. It is not unusual for rowers within a shell not to agree on what needs to be done to establish a `good' set, i.e. a level, stable shell that will provide the basis for that symphony of motion.
Check
Any abrupt deceleration of the shell caused by some uncontrolled motion within the shell; an interruption in the forward motion of the shell. The coxswain is probably the most acutely aware of this abrupt deceleration and it has been known to cause whiplash in some extreme cases.
Crab
A problem encountered by a rower when his or her oar gets `stuck' in the water, usually right after the catch or just before the release, and is caused by improper squaring or feathering. The momentum of the shell can overcome the rower's control of the oar. In more extreme cases the rower can actually be ejected from the shell by the oar.
Jumping the slide
Another problem encountered by a rower when the seat becomes derailed from the track during the rowing cycle.
Missing water
The rower starts the drive before the catch has been completed (or even started in some cases). This is also referred to as rowing into the catch.
Skying
The fault of carrying the hands too low during the recovery especially when a rower dips his or her hands just prior to the catch (i.e. a sort of winding up). This usually results in the blade being too high off the water's surface.
Washing out
The fault of rowing the oar out of the water, i.e. the blade comes out of the water before the drive is finished.


Race Formats

What are the usual racing distances and divisions?

The races have separate divisions---Men's (M), Women's (W), heavyweight (HWT) or open, lightweight (LWT) etc., then divided up into 8+'s, 4+'s, 1x's, 2x's and so on. So for a typical regatta you might see separate races scheduled for M8+, W8+, M4+, W4+ down (or up---depends on your cup of tea) to W1x and M1x. There may be separate heavyweight and lightweight divisions that would require a weigh-in for the lightweights some time before the start of the regatta. You may also see divisions according to experience (novice, varsity), age (junior and masters) ,and skill level (senior A, B, Elite, etc.)

Standard

The standard international racing distance is 2000 meters (preferably straight) in their separate designated lanes which may or may not be marked by buoys. These races can take anywhere from 5 1/2 to 8 1/2 minutes depending on boat class, weather conditions, water current and the physical condition and experience of the rowers.

Other racing distances are 1000 meters for the older men and women (Masters) and 1500 meters for the Junior age division (high school). A description of the starting procedures is in a separate following section. Also, there is a match style (i.e. races with two boats head to head in a single elimination format for each division) racing at a some regattas.

Head Races

These races , which are generally held in the fall (US) or early spring (Europe) are about 2.5-3 miles long and the boats are started in their respective divisions separately at 10 second intervals. These things are usually conducted on a river with an assortment of bridges and turns that can make passing quite interesting.

Bumps (This is the Cambridge version, but it should do just to get an idea of what bumps racing is about.)

The bumps are a way of racing eights. It all basically comes from rowing on a river which in most places is only just wide enough for two boats to pass.

The basic idea is simple: you get a division of 17 (or 18) boats who start in a column with 1.5 lengths of clear water between them, and when the start gun goes the aim is to bump the boat in front by making up enough distance for physical contact between the two boats. The two boats involved in the bump drop out of the race by pulling in to the side of the river and leaving the course clear for anyone behind (if the boat behind a bump catches the boat in front of a bump this is an overbump).

In the next day of racing the two crews swap start positions. There are 4 days of racing in each set of bumps, and positions are held over from year to year. Divisions are raced in reverse order (i.e. worst first) and the crew ending top of a division (because it started there and successfully `rowed over' the whole course, or because it bumped the crew who started head (top) of the division) gets to row as the 17th boat in the next division so if they bump there they move up a division the next day. The aim of the whole thing is to end up top of the 1st division `Head of the River', or to go up four places (i.e. a bump each day).