SDK Versions

The Source SDK is the set of programs that allow you to create mods, maps, and just generally "develop" using Source.

The Source SDK Base is the set of required files to play any mods created using the Source SDK. For example, if you only want to play Source mods, you'll only need the Source SDK Base. But if you also want to develop mods and maps, you'll need both the Source SDK and the Source SDK Base.

The Source SDK Base 2007 (also known as "Source SDK Base - Orange Box") is the set of required files to develop and play in mods using the Orange Box version of the Source Engine.

Since Valve released the Left 4 Dead Authoring Tools instead of updating the Source SDK for use with Left 4 Dead, there is no "Source SDK Base - Left 4 Dead." Unlike the Orange Box Source SDK, the Left 4 Dead Authoring Tools are only for creating maps for Left 4 Dead, which has all the files required to play the maps (hence the lack of Source SDK Base - Left 4 Dead). This is also true for Alien Swarm, Portal 2, and Counter-Strike: Global Offensive.

In 2013, Valve released Source SDK 2013. This doesn't work with the previously-mentioned games, but it comes with new Source 2013 features like Linux support. Additionally, all Valve games on earlier branches were updated to Source 2013 Singleplayer or Source 2013 Multiplayer, as appropriate, so Source SDK 2013 has improved compatibility with current versions of those games.