Does steam have an SDK

Some games on Steam do.
These games include:
Day of Defeat: Source
Left 4 Dead 2
Team Fortress 2
Half-Life 2
Portal
Half-Life 2: Episode 2
Counter-Strike Source
Alien Swarm ( Free on Steam)
Half-Life 2:Episode 1
Left 4 Dead
Half-Life 2: Deathmatch