How Transistors Work

[Alan]

"A Transistor can be thought of as a device that is active in only One Direction: It can draw more or less current through its load resistor (sometimes referred to as a pull-up resistor). It can either Sink Current or it can Source Current, it Cannot do Both."

http://www.williamson-labs.com/480_xtor.htm#introduction

Via:
http://www.makezine.com/blog/archive/2006/06/how_transistors_work.html