It is just like a forum but more interactive (duh!). Everybody can create a chat room on S.O. - but the meaning of is determined by participation - so say hi, ask and answer questions there. Initially it may seem as semi-offline mode, as people are not simultaneously logged in - but eventually we may start gathering around specific times based on stats, it's early to plan.
Here is the URL - just click to open and you are "there":
Needs only a web browser to chat. History persists (unlike IRC) and is public. Can use Markdown for formatting. Can opt for desktop notifications on some OS. Details on the "hows" and "whys" of said chat system can be found in the FAQ.
StackOverflow account and a minimum of 20 reputation is required. If you don't have account - that's trivial to do (can even use Google/Facebook/Yahoo/blogs login) and start using, the karma is easy to come. If you don't even know what StackOverflow is yet though... i am afraid i won't be able to help, my apologies.
See you in the Roku room of http://chat.stackoverflow.com