Most confrontations in Zero-K will last for about a half-hour, with your objective being to finish off the rest of the bands out there (both human and artificial intelligence) so that you have total control of the map.
You'll start each game with just one unit: the commander. From there, you'll need to collect resources (metal and energy) so that you can construct a series of factories to build all kinds of new units. There are more than fifty types in all, with land robots, ships, airplanes, and more.
Zero-K includes various playing modes. There's the typical melee mode where you'll face off against other users over the Internet, as well as 'Chikens' mode, and the persistent world of 'Planet Wars'. Both are great and get you a lot of extra mileage out of the game.
Zero-K is one of the best games out there in terms of real time strategy. Not only is the game totally free, but there are almost always players connected, so you won't have any trouble finding games to play.