Source is a game engine created by Valve Corporation, and released in 2004. It powers games such as Half-Life 2 and Portal. It's also the successor to the GoldSrc engine and the predecessor of Source 2.
Articles directly under the Source umbrella describe glitches, tricks, and mechanics possible in multiple games running on the engine.
Click here to browse all articles relating to the Source engine.
Just like the GoldSrc, Source is a great engine for various sorts of modifications.
But what is... a mod?.. I sure don't know it myself.
While the distinction between a standalone game and a mod can be vague and baffling at times (primarily on Steam), the main traits of a mod are:
Usually, identifying them isn't hard, since most mods are placed in the sourcemods folder and require a specific SDK Base to run.