As you may have read in this thread Xfire is the premiere chat platform for gamers.
When you are at your desktop it functions similiary to AIM. With the added functionality of being able to track how long you spend in games, and moniter what games your friends are playing as well as what servers there on. Xfire also keeps a profile on you for example here is mine: My Xfire Profile
When in game with certain games including RvS Xfire supports in game messaging. A small bubble will appear in the bottom right hand corner of you screen in RvS when you have a message. If you would like to respond to the message you hit a hotkey and an IM like window appears on top of RvS without and minimizing or screen refreshing. you can than type your message and go back to gaming. The IM system works between anyone else on xfire reguardless of what game there playing or if there not playign a game at all.
So If you choose to get it list your xfire s/n here