The DLC for Modern Warfare 3 was released in four major "Collections," which are still the primary way to acquire the extra content on PC:
On PC, the matchmaking system prioritizes players who have the same DLC configuration. If you have all DLCs enabled, you will primarily be matched with other DLC owners, which can sometimes lead to longer wait times in older titles.
Includes three Multiplayer maps (Sanctuary, Foundation, Oasis), two Special Ops missions (Kill Switch, Iron Clad), and the debut of "Face Off" maps (Getaway, Lookout).
Having all DLCs installed will significantly increase the game's footprint, adding roughly 10-15 GB to the base installation.