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 application is now deprecated in 
Source 2013, as most of these games now ships with Source SDK Tools by default, but without SDK Launcher.
Source SDK Base and versions
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 latter also prevents your mods from not working due to the game being updated, which makes it incompatible with your mod built on older versions of Source.
2006
The original version of Source SDK Base is now referred as Source SDK Base 2006, required to play any mods built on Old Engine (Source 2006). It was last updated in December 2009.
2007 (Orange Box)
The Source SDK Base 2007 (formerly known as "Source SDK Base - Orange Box") is the set of required files to develop and play mods built on the Orange Box (aka Source 2007) version of the Source Engine. Last updated in Feburary 2010.
L4D branch, Alien Swarm or above
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 Source SDK (2006, 2007 and 2013), the Left 4 Dead Authoring Tools are only for creating maps and some content 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. Authoring Tools for these games usually didn't include the source code for these games (except Alien Swarm). Additionally, there is no Source SDK Base 2009 either.
2013 (SP/MP)
In 2013, Valve released Source SDK Base 2013. This version is incompatible with mods made on previously-mentioned versions (like SDK 2007 and older), but it comes with new Source 2013 features like Linux (and better cross-platform) support, 
custom folder for much easier custom contents installation. 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.
The SP branch was last updated in 2014, and does not feature any important bug fixes in Half-Life 2's Steam Deck and Anniversary updates (such as the Dropship gun turning fix). The MP branch was last updated in 2021 before it was replaced with Team Fortress 2 branch version of SDK 2013 MP.
TF2 branch (2013 MP v2)
In 2025, Valve updated Source SDK Base 2013 - Multiplayer, which is now based on 
Team Fortress 2 branch, featuring major changes including one from Half-Life 2 20th anniversary update (radial fog, bicubic lightmaps). This version is incompatible with mods built on Source 2013 Multiplayer, instead of Team Fortress 2 branch. All Valve games on Source 2013 Multiplayer have also been upgraded to this branch, completely replacing Source 2013 Multiplayer. This SDK Base is actively being maintained (as of Feb 27, 2025).