Dexter is a serial killer who cleans up all the murderers that the police system can't process because of loopholes in the system (illegal search warrants, tainted evidence, technicalities etc). He works with the police department as a blood spatter analyst and uses his position to get the info on all his victims.
It's quite a good watch, well written, brilliantly cast and keeps you wanting more and more. I slammed through the first two seasons in two and a half nights. Had no sleep and was functioning solely on V and coffee at work, total autopilot.
Season 3 was a one nighter, and Season 4 I just did over the last few days as a couple had to be downloaded again. I was not a patient person while waiting for them to download a second time... there was quite a lot of "The chili cookoff is happening right now! and i'm missing it!" dancing going on.
Worth checking out at least the first season if you've got the time.