It's the best tactical shooter you'll ever buy. Buy it play it. Now.
You want more!? I'll give you more...
Frozen Synapse is a turn based tactical shooter. You control a small team (up to about 5 soldiers) and you play in 5 second turns. Don't worry, you get to spend much longer than 5 seconds planning them.
Each turn, you can give each of your units orders, telling them where and when to move, aim, duck or stand. There are also more subtle controls for toggling your soldiers to ignore or engage targets and for focusing on a single soldier.
I'd write more, but I'm tired. So I'll link to the fine article my beloved RPS wrote on it.
Really good article by Kieron Gillen
For a more mechanical description of the game, see this forum post.
Seriously. Buy it now, play the tutorial then challenge me to a game (my username is DrazharLn).
P.S. Game is in early beta, you're preordering it and getting access to the beta and all consequent releases, much like Mount & Blade's distribution model.
P.P.S. Game is in early beta, adjust your expectations accordingly.