A little black lump of silicon whose invention represents the 'opening shot' of the entire concept of neato home electronics.

The average transistor has three legs, protruding from the underside of a black semi-cylinder. The cylinder is about five millimetres high.

What does it do?

A transistor is a lot like a switch - you can control the flow of electricity across two legs by changing the input into a third leg. But another important property is amplification - a small change in the input on the controller-leg makes a big change in the flow across the other two legs. This is the basis of pocketable transistor radios - taking a very weak signal in the air and amplifying it until you can hear it through speakers.

The three legs are called the Base, the Collector and the Emitter. Imagine the Collector and the Emitter being the 'in' and 'out' of our virtual-switch. The Base is the controller for that switch.

Which leg is which?

Looking at the flat face of the cylinder, legs hanging down, you'll see three legs, in this order: Left, Back, Right.
The standard dictates that they are:
Left: Collector
Back: Base
Right: Emitter
Now you know.

What can I do with it?

Well, a fun thing to do with a standard NPN transistor is to make a touch-sensitive light. (Thanks for idea go to http://www.halpounds.freeserve.co.uk/Electronics/Tutorial/)
All you need is an NPN transistor, a battery, and an LED.

1. Connect the LED in a normal circuit with the battery.
2. Place the transistor in the circuit like a switch for the LED (i.e. in series with it): Collector towards positive, Emitter to negative.
3. Touch the positive battery terminal with one finger, and the Base of the transistor with a different finger.

The LED should light up when you do this.
This is because the small amount of electricity passing through your body (yes, we conduct slightly) from the battery to the Base is enough to make the transistor allow a large current through, lighting the LED. It amplifies your conductivity.