DLC is short for Downloadable Content, and is posted by Day 1 Studios and MS, the developers of Mechassault to enhance an already great game.  So far there have been two DLC's.  The DLC will usually include improvements to the game by offering new Mechs, maps, game types, and makin the game more stable.  Details of the two released DLC's are below

DLC number one included two new Mechs and two new maps.  The two Mechs were the Raven and Corvus, both fast light Mechs designed for use in the new game type of Capture the Flag to come in the second DLC.  The two maps released were Stone Cold and Hell's Kitchen.

The second DLC was released just recently and updated the game to make onlline play more stable and gives the option to boot undesirable players from the game (YES!!!!).  It also introduced the new game type of Capture the Flag, a game type available only for multi-player games.  There was also a new weapon, the Flamer, released.  Along with this were released two new Mechs, the Loki and the Hellbringer.  There were two maps that have also been released , Midtown Mayhem and Desert Storm.  Both of these maps are available only for CTF play.

Both of these DLC's were free, but it is not currently known if future DLC will be free or pay.