If you have a few hours to burn, please read
http://sound.westhost.com/amp-basics1.htm
It is easily the best intro to amplifiers that I've ever found. It assumes no prior knowledge of electronics and runs all the way up through the full history of amplifiers. You will understand amplifiers in a way that will put you years ahead of your classmates when you are finished. I can not praise this enough.
OR, the short answer:
A small amount of current into the base allows a larger amount of current into the collector. The ratio is called Beta, and is typically on the order of 100. That is, if you put a 1mA signal in, you can get a 100mA signal out.
Take a look at this figure:
http://sound.westhost.com/ab-f2-3.gif
Write appropriate scales on the axes, and you are good to roll.