Forums → Support Forum → Steam ID

It's a gaming community that hosts games and sometimes creates games for people to play online. It also has mods of other games. I guess you could compare it to other gaming servers like battle.net

Thanks for telling me.

To be more precise:

It is the company that 'made' Counter-Strike, created Half-Life, etc..

The Steam ID is used for multiplayer games online.

You can also purchase game online. Steam is very useful because you can see what games are playing your friends and join it. Oh and Valve made CS, not steam.