In brief, it's a puzzle game where you combine elements in reactors to create different products. If you like puzzle games at all then you owe it to yourself to get this game. It's one of those fantastic things that's simple in premise, yet gets incredibly challenging.
View the videos and stuff, and you can play the demo first also.
http://store.steampowered.com/app/92800/?snr=1_4_4__13
Also, one of the prizes in the Summer Camp promotion is SpaceChem DLC, so you can get these bonus missions completely free!
I seriously cannot recommend this enough if you're into games like this at all.