It's a running program that slowly eases you into running. I think it's 8 or 9 weeks long. So you run three times a week. Each week you increase your running intervals and decrease how much you walk. So week one, you run 60 seconds and walk 90 seconds. By the end of the program you should be able to run a 5k! Hence couch-to-5k, because it's great for people who have never run or are not fit.
I have an iphone application which I use during the runs. It lets me play my own music, but tells me when it's time to run/walk. Very useful!
If you google couch-to-5k there is A LOT of information. They have a facebook page too which is a great support system.