The armies are selected before the game, up to a pre-arranged points quota. From there, each player uses his or her "Codex" (consider it a sourcebook for the given army) to select units, characters and upgrades he/she will use.
In addition to the points value (and a cap on maximum value of equipment a single character can use) those choices are also restricted by a "force organization chart" which puts restrictions on maximum number of units of each class and pre-requisite number of units from specific classes that must be used.
Then there is the actual game, in which there are several types of scenarios, with varying special rules and victory conditions.
In some at the end of the game (either after a pre-defined or random number of turns) the player who kills more of enemy units in terms of their points value is victorious. In others it's based on controlling table quarters or specific objective markers, which are the closest thing to the control points in the system (except there aren't normally any more than six of them - usually as few as three, actually). Others yet base the victory on controlling a specific terrain feature. And so on. There's a bunch of examples at
http://macragge.uk.games-workshop.co...os/default.htm , if you like.
(Can't pass an opportunity to convert another unbeliever to the way of The Hobby, can I? :laugh
