Pretty soon you'll see a Facebook chat bar at the bottom of your screen whenever you log into Facebook, you won't need to do anything. With the Facebook chat bar you'll always be able to see which of your Facebook friends is online. Then you can start a chat with one of them, or all of them at the same time if you want.
The chat window can pop out into it's own window so you can chat and do other things on your computer at the same time. Just like other IM programs, you'll be able to chat with your friends in real time with Facebook chat. You won't need to set up a buddy list though, Facebook does that for you from your Facebook friend list.
Read a first hand account of Facebook chat from Justin Smith of Inside Facebook. He's been allowed to give it a trial run as they slowly release the new chat feature.