At this moment you can play Spring on Windows and Linux, we still need someone to finish the Mac port.
To play Spring you need a few things:
The Spring engine, it renders the screen, moves the units, calculates the projectiles, handels your orders and keeps you in sync with the other players. It's just a lot of thechnical bits and bytes.
Units that you can command, these packages are called mods or games.
Maps wich are the worlds you whage your wars in.
And a lobby, with this you can set up a battle agains others.
Optional is an AI (artificial intelagense) for those that like to play by themselfs.

The base Spring installer/package includes enough to get you started, but there is a lot more to be found if all four, so you should