The Buddy List is a standard feature of RMRK's forum system; SMF. By default, it is not very useful at all, though it got a lot more attention after Roph made an edit to the profiles so that (in a rather myspace-ish style) they showed members who were people's buddies on that user's profile page, however, that feature is no longer present due to incompatibilities with recent versions of SMF.
There was an unfixed bug that sometimes caused all the columns to be squished up together. Also, members with avatars that were wider than the maximum (only possible by them having an avatar that was wider before the limit was introduced and having not changed it since) would mess up the display a little. The system exposed Winged as being a special member, as if she was on certain people's buddy lists, when viewing their profiles, her name and avatar would be displayed as much as ten times.
The Buddy List has other uses. Members who are on your buddy list get their names shown in bold when viewed by you. Also in your own profile admin pages, there is a neat page that will list all of your own buddies for you and show you their online/offline status, along with all of their contact info.
After upgrading to SMF 2.0, the buddy lists on profile pages no longer functions, nor does it show buddies' names in bold in the Users Online list. As such, buddy lists have gone back to being fairly useless.