The crossover will deal with the Pheonix force (Not sure if we'll see Jean come back or not) and also reportedly the recently redeemed Scarlet Witch (Who has a history with both teams).
Of course this isn't the first time the team have crossed over. Apart from Scarlet Witch and Quicksilver who at that point were X-men villains joining the team early on, Beast joining the team for a while and Rogue's complex history with Ms. Marvel (Who herself was an X-men supporting character for many years). There also was a miniseries where the two fought over Magneto joining the team in the 80s, problems during the Secret Wars, Onslaught , Bloodlines and of course the House of M.
Of course the major reason for this is probably to promote the Avengers film