Marvel Team-Up was a comic book published from 1972 to 1985. Now it's a feature in Marvel Heroes.
Developer Gazillion said yesterday that Team-Ups will let players fight with two heroes against supervillains. Four heroes will be available in the first wave: Falcon (who made three appearances in the comic book); Magik of the X-Men (and Colossus' sister); Firestar, created for the 1980s TV show Spider-Man and his Amazing Friends; and, of course, Spider-Man himself, who starred throughout Marvel Team-Up's 13-year run.
Team-Ups will offer three different playing styles, allowing them to fight alongside players or protect them from off screen. The heroes all have their own distinct power sets.
For more information, see this FAQ.