There are really two questions here... what does a transistor do... and HOW does it do it.
To get a look at the what does it do, try using my favorite circuit simulator
http://www.falstad.com/circuit/ and take a look under the "circuits" menu item for transistors and then choose PNP or NPN transistors as appropriate. Play with the sliders on the bottom right, and watch the current flow.
You'll find other, more complex, circuits in there to check out and... when you get the hang of it, can modify them or even build and simulate your own circuits. So even those who already know what transistors do... it is worth your time to check the software out. Easily one of the coolest java applets that I've come across.
Now, as for the "how does it do it"... well, if you just said "it uses magic smoke and I'll worry about the rest of it when I have to", it would be completely forgivable. The first thing you need to know is how a diode works. My favorite diode explanation (skip the first minute of it... it is really good after the first minute...) is on youtube at
http://www.youtube.com/watch?v=CvRZG1zL2o0
Once you've got the idea of a PN junction diode and doping then you can understand what a PNP or NPN junction might be like. But most diagrams of transistors aren't really clear about the fact that the two outer layers are doped slightly differently from each other and that the middle layer (the base) is really, really thin relative to the other two layers. A simulation is online at
http://www.learnabout-electronics.or...sistors_04.php
Hopefully that helps your understanding.
Jason