It's a microcontroller board that you can program easily using BASIC computer code. Google 'Basic stamp' to find out more.
I use Picaxe, which is quite a bit cheaper than the stamps, and just as versatile. Also supported by one of the best forums I have encountered on the internet.
What can you do with one these gadgets you might wonder. Well you can control multiple servos off a single channel, flash lights on and off, speed control motors, add timers, failsafes, torpedo/rocket control- the only real limit is your imagination.