Counter-Strike: Source is an online only game that was released on Steam and in retail stores on November 1st 2004 for the PC.
On June 23 2010, Valve released it onto Apple computers through Steam. Although on separate platforms, the players can play each other through the same servers.
The game runs better on PC, with higher frame rates and better textures, but it still runs decent on Macs.
As stated earlier, CS:S is online multiplayer only, with support for dedicated servers. It allows for custom maps to be uploaded and played through it's SDK that was released in 2007. Also, server hosts can run custom gametypes, and utilize custom skins and sounds, that are downloaded by a player when joining a server for the first time.
To prevent cheating, it also uses VAC (Valve Anti Cheat) to protect it against hackers. If the server has sv_cheats 1 (enabled) then the player can use the developer console (~) to achieve many feats such as noclip and running speed changes.
Alongside this, the Source engine that the game runs on, has the demo feature built in. Players can record a demo, which is a saved file with all the game assets, physics calculations, and score recorded. This is an early version (and more robust) version of the Theater in Halo 3 and Reach. It allows for recamming clips, keyframming, and much more. Also, the engine can create flawless video by making each game frame into a seperate lossless JPEG that can compiled into a video, which makes beautiful recordings.