X-Factor is an X-Men spin-off that originally featured the original X-Men team. For several years they existed as rivals of the ongoing X-Men team, which was led at the time by their old foe Magneto. Eventually this team was re-incorporated into the X-Men during an expansion of the franchise.
Following the departure of the original group, writer Peter David began a long association with the feature. David has written several incarnations of the team with various premises. X-Factor usually exists independently of the X-Men team, but is generally comprised of characters who have been closely associated with it or another spin-off in the past, such as Quicksilver, Havok, Polaris, and Wolfsbane.