It is called hundredpics, and it's pretty simple. You take a picture a day and post it for 100 days. If you skip one you start over.
He just created it today, so it doesn't have any rules yet. I don't know if you have to do your posting on the same day or if you just have to take the picture each day. I'm planning to do it at some point but not right now. This is a horrible time of year for me photographically because it is dark when I get to work and it is dark when I leave. bleah. When the world lightens up I'll give it a go.