From Valve Developer Community
A modification, often abbreviated to mod, is an alteration or creation of files for a game engine, which allow it to modify the gameplay style, graphics, environments, models, etc.
Types of Mods
- Third-party modifications are mods that has not been developed by the original game designer. Mods are typically third-party, as first-party mods are often called expansions.
- Total conversions (sometimes abbreviated TC) are mods that have altered their original games to such extents (through new models, new animations, new code, among other things) that their original games can no longer be recognized. The amount of work and expertize required for total conversions, makes these types of mods very rare.
- Dead modifications are mods that (often due to loss of interest or internal problems) have ceased development and been abandoned without producing a release.
To install a modification manually, follow these instructions:
- Copy the mod folder (with the gameinfo.txt file directly inside) into your sourcemods\ folder. (<Installation path>\Steam\SteamApps\sourcemods\, for example C:\Program Files (x86)\Steam\SteamApps\sourcemods\.)
- Start/Restart Steam. If the mod is available for play, Steam should list it in your Games library.