The fan belt in your car is the belt that turns the fan. END OF WRITEUP.

Well, no, obviously there is more than that. The fan belt is typically responsible for the horrible screeching noises that occur when you start your car and your fan belt is loose. Fan belts wear out, and as they do this, they become longer. This necessitates tightening every so often. Fortunately, tightening of the fan belt on an older car (one that doesn't have a serpentine belt, in other words) is a rather easy proposition if you have the properly sized wrench, which of course you have as all car owners have a full socket set with both metric and Imperial unit sockets. Right.

In my old car, the fan belt was easy to find. As you were facing the vehicle with the hood open, it was on your left, at the very front.

To tighten your fan belt
1. Check for wear. You should be able to turn the thing over to see if it's cracked, pitted, broken (!), or about to fall apart.
2. If it's worn, you should replace it or have it replaced.
3. If it's not, you can tighten it by unscrewing the large thing to which it is attached. (probably the one closest to you) Slide it so it tightens the belt, then tighten the bolt that you unscrewed earlier.

Fan belts are cheap enough that you can and should keep a spare one in your car. For moxie's car, it would cost about twelve or thirteen US dollars. If it breaks while you're on the road, you're not going to go anywhere soon, as your car will very quickly overheat, and your battery will die, as the belt often turns the water pump and alternator as well (thanks Kidas and stupot for the advice.)

Your local public library probably has Chilton's manuals or something similar for your car. I suggest you take a look to make sure you know where everything is.

"The fan belt in your car is the belt that turns the fan. END OF WRITEUP.". On most modern cars, this isn't the case.

On older cars, the fan belt did drive the engine fan, the main fan that blows air over the radiator to help cool the engine down (this is separate to the fan that blows air into the cabin to keep you hot or cold).

However, this means that at low speeds, in start-stop traffic, when the engine cooling needs to be at its best, the fan is operating slowly, and at high speeds (eg on the motorway) when the air rushing through the engine is enough to cool it by itself, the fan is operating unnecessarily. Additionally, it means that as soon as you start the engine, the fan is cooling it, at a time when you want the engine to warm up as quickly as possible!

Most modern engines have electric engine fans that are driven by a simple motor off the battery. Rather than running all of the time, they are turned on and off by a thermostat when the engine is getting too warm. This will almost never happen at high speeds, even on a hot day, but will often happen when in slow traffic on a hot day. You can usually hear it starting up and stopping (and it sometimes will run even once the engine has been turned off until it cools down). Some cars even have two engine fans.

That's not to say that cars don't have a fan belt, just that it's a bad name for it. At the least, the fan belt connects the engine to the alternator, and additional "fan-type belts" connect it to the power steering and air conditioning compressor if they are present.

This all notwithstanding, all of mkb's comments on checking, tightening etc the fan belt apply to these other belts.