Counter-Strike: Source has been replaced with Counter-Strike: Source Beta on my steam account. Does anyone know why and what is different about it?
It hasn't been replaced. Counter-Strike: Source is still there, but Valve opened up the beta once more for everyone.
As for the changes, it seems nothing visible has been done yet, only bugfixes and performance testing.
They did this to test certain new features to the engine without breaking the main game, a valid concern for one of the most popular multiplayer games of all time.