It was a manga series, and then they turned it into a show. I read it AND saw the show because I'm a loser.
It's about this dude that finds a notebook that was purposely dropped by a shinigami. The notebook (called a death note) allows you to write a name in it, and then that person will be killed. So the dude decides to use it to kill criminals. He decides that he will cast judgement and become the "god of the new world".
It's really good. It's the only anime I could really get into.