Source is the game engine on which Half-Life 2 (and several games related to it) runs on.
Source uses a modular code system, so certain elements like a rendering pipeline or VGUI2 component can be removed, replaced, and reworked easily. It also features an advanced texture and material system which makes it easy to define advanced shaders and properties on materials.
Notable games running on the Source engine include:
Half-Life 2: Episode One
Half-Life 2: Episode Two
Half-Life 2: Deathmatch
Half-Life Deathmatch: Source
Counter-Strike: Global Offensive
Left 4 Dead
Left 4 Dead 2
LEFT 4 DEAD -生存者たち- (LEFT 4 DEAD -Survivors-), a Japanese arcade game