Background

Since the early days of the internal combustion engine, there has always have a device which was known as the throttle body. In an automobile, its sole task is to respond to pressure on the accelerator pedal, open, and allow air to enter the intake manifold so that it could be combined with fuel to provide energy.

Despite this simple function, a throttle body never was, nor is, a simple device. It's by no means as complex as most parts of the valve train, but nonetheless, it isn't lacking in intricacies. Here is a diagram of a typical throttle body.

Diagram

                                                                                                                                                                                                        
                                                                                                                                                                                                        
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Components

A modern throttle body assembly generally consists of the following parts:

Body of the throttle
A machined piece of metal, generally aluminum, which all the other parts attach to. This makes up the bulk of the throttle body, and from this point on, when I say "throttle body", I'll be referring to this piece.
Throttle position spring
One side is attached to the throttle pivot. The other side attaches to the throttle cable. In the diagram, this is on the left side of the throttlebody.
Throttle plate
In its default position, it blocks off the air passage in the throttle body. When the throttle plate is wide open (WOT), air is given almost no resistance and allowed to be sucked into the engine. The throttle plate is not visible in the diagram, since it is in WOT position.
Throttle pivot
The throttle pivot is positioned in the middle of the throttle body opening. It rotates as the throttle position spring moves. As it moves, it affects the throttle position sensor.
Throttle position sensor (TPS)
Attaches to the other side of the throttle pivot. On the diagram, it is on the right. This is basically a resistor which alters its current depending on the rotational position of the throttle plate pivot. This sensor tells the Electronic Control Unit (ECU) of the car how far open the throttle is at all times.

There are sometimes vacuum passageways or coolant bypasses on throttle bodies, but those aren't universal, and aren't important to understand how one functions.

Function

Your foot presses the accelerator.
The throttle cable pulls on the throttle position spring.
The throttle position spring rotates the throttle pivot.
The throttle plate moves open in accordance with how hard you push the pedal.
The throttle position sensor sends data to the ECU, telling it how wide the throttle is open.

At this point, the throttle body's job is done. Depending on what type of airflow sensor the car uses, it will then determine how much gasoline to tell the fuel injectors to send.

Maintenance

Occasionally, lubricate the throttle pivot. This keeps the opening and closing actions smooth, and improves throttle response.

The throttle body, despite being fed air that has already passed through the car's air filter, still gets dirty with time. Every 25,000 miles, you should remove the throttle body and clean it with an abrasive agent. Most auto body stores will carry throttle body cleaner, in easy-to-use spray form. In a jam, brake cleaner will do fine, but be sure to wipe off the excess before putting the throttle body back in place.

Please consult a good automotive manual, such as a Chilton, in order to find out the proper procedure for removing your throttle body.

Performance Tips

The throttle plate is nothing more than a thin disc of metal. However, the blunt edge of the disc can create minor airflow disturbances as air passes by it.

You can lessen the turbulence in the airflow by knife-edging the leading edge of the throttle plate. The leading edge is the edge that opens towards the air.

If any other performance modifications have been done to the engine, such as a more aggressive camshaft, a cylinder head port and polish, or cylinder over-boring, the engine may be capable of breathing more air. By obtaining a larger throttle body, you'll ensure that it is not the bottleneck in maximum performance.

Replacement

The throttle body is composed of many simple parts that form a semi-complex whole, and as such, it never will fail as a whole. Unless you recently took a sledgehammer to your engine, your whole throttle body will not need replacing. Any mechanic who tells you that REALLY means, "We can't order just the part that failed, so we need to order the whole thing. Since we've ordered the whole thing, we might as well install it on your car."

A spare throttle body from a junkyard costs less than $10 to $20 (USD). A new unit from the manufacturer costs anywhere between $75 and $200. I personally recommend picking up a spare one from the junkyard, which gives you an extra of each part, so that if one fails, you won't end up getting slapped with an exorbiant amount by the mechanic.

Common things which go wrong on a throttle body:

If the throttle position spring breaks, that would need to be replaced. If you've got that spare throttlebody, you can hand that to your mechanic and have him cannibalize the part off of it. Replacing the spring should take a skilled mechanic no more than an hour and a half.
The throttle position sensor, being electronic in nature, could fail. These are expensive all by themselves, so once again, having a spare one on hand will greatly minimize your out-of-pocket repair costs. This operation takes about the same amount of time as replacing the spring.

There isn't much else that can go wrong with the throttle body assembly itself. Some throttle bodies house a coolant bypass, but those tend to be detachable units and aren't standard. Again, consult a manual such as Chilton's if you're unsure about the specifics of your throttle body.

Resources

The top of my head.

http://everything2.com/title/throttle+body

http://everything2.com/node/1343885