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 making 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 online 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.

There has been a third release of DLC recently, this includes three new maps and five new game types.  The new maps are entitled Demolition Town, Rock Solid, and Jottunheim.  The first two are available for use in all game types except CTF, while Jottunheim is available only in CTF.  The five new game types are available only for multiplayer games and include Scout Match (where you are limited to only using light Mechs), Team Scout Match (same as Scout Match, except you use teams), Skirmish (all Mechs except assault Mechs are available), Team Skirmish (same as Skirmish, except you use teams), and Giant Killers.  Giant Killers is a variation of Not It, and makes use of a popular game type that while not official, was played by many players called Godzilla.  In Giant Killers, everyone picks a Mech in the lobby and when the game starts, the It player is their chosen Mech while everyone else is an Elemental.  When the It player is destroyed, then the player that destroyed is respawned as their chosen Mech.

In addition to this as part of the DLC, the issues with drops and the Optimatch choice seem to have been resolved.  There has also been another DLC that allows players to play on maps that were previously not available for use in all game types (Number 51 for TD, OH YEAH!!!).