A Half-Life MOD is a modification that someone has made to the single player or multiplayer version of the game Half-Life. These MODs usually incorporate new weapons, new levels (maps), and/or new methods or rules for playing the game. Some MODs are single player only, which means that you play against monsters (like in the original Half-Life game) using new weapons, new levels and/or new monsters. Other MODs are multiplayer (deathmatch) only MODs, where you play against other human players connected through the Internet, or on a Local Area Network (LAN). Some MODs support both single player and multiplayer.