The buddy list is used to keep track of the friends you have made on this forum. By going to your "Open Buddy List" in "Quick Links" menu of the navbar, you'll be able to see which of your friends are currently online and send them a private message. Adding people to your buddy list also allows you to send private messages to multiple forum members at the same time. You may add any member of the forums to your buddy list by viewing their profile or by going to "Buddy / Ignore Lists" in your "User CP".
Ignore lists are used for those people whose messages you wish not to read. By adding someone to your ignore list, those messages posted by these individuals will be hidden when you read a thread.