It’s a cross between a programming web forum, Wikipedia and Digg. Basically people ask questions, people answer questions and people vote on the answers to push them higher or lower in the stack. If more clarity is needed the original question and answers can be edited as needed rather than posting new responses. Theoretically this means the good answers will float to the top and they will be complete answers so you don’t have to read through dozens of responses to figure out the complete answer. It also means that old, obsolete questions and answers will tend to sink to the bottom of the stack. Also, questions and answers can be tagged so you can do more targeted searching. It’s free too (no Experts Exchange bait and switch tactics).
You can read about the idea behind it here
The more time I spend on the site the more impressed I am with their ideas. As you post questions and answers and those posts get voted up or down you gain and lose reputation. At certain reputation levels you get more power on the site (ability to vote posts up/down, ability to comment, etc). It really should cut down on the noise since new users can't comment at all and they can't vote the questions/answers up and down until they've spent some time on the site. That means, hopefully, it will be harder to "game" the site like people try to do with slashdot and digg.
Check it out, it's going to be a killer site. They should branch out into non-programming sites too.