There are six factions competing for control of the planet Dune; the Emperor, House Harkonnen, House Atreides, The Guild, the Bene Gesserit, and the Fremen. There are a certain number of cities on the board, and capture of either three or four (depending if you are attempting to win singly or as part of an alliance with another player) signals the end of the game. There are a limited number of troops that each faction can deploy, and combat is handled with a combination of troops sacrificed, leaders used, and spice (money) spent. Cards can be played in both combat and other situations, which gives a nice randomness to the flow of the game, but one which is mitigated by some of the special powers each faction possesses. Spice (money) appears randomly in certain spots on the board, and some factions are in a better position to collect it, either by virtue of their special powers, or by which cities they hold.
Perhaps the most devastating power is the Bene Gesserit's ability to predict the winner. By clever and subtle play, they can manipulate the player of their choice to win on a particular turn. If they predict successfully, they win instead! I've seen it happen more than once, and it's devastating when you're the one they've predicted.
In the course of preparing this post, I did stumble across this web site, which might be of interest to those looking to see the details about the game, with images of the cards and other pieces, a copy of the rules, the board, etc. Ahem.
There is, however, an issue of The General with alternate rules for a seventh faction; the Bene Tleilaxu. I've played with those, and they work really well, with the BT being very well balanced in the tradition of the original. You'll also find that on Colin's Dune Page, linked above.