Just like Starcraft, the goal is to destroy your opponents.
There is a ladder.
There are around 30 different types of units, each with strengths, weaknesses etc.
Different armies cost more to produce, more for upkeep, and take more turns to complete.
The game revolves around building poweful stacks. Up to 8 units can be grouped into a "stack". Many units give all other units in the stack a bonus. So good stacks can destroy exponentially more than they lose if they are built well. It's not uncommon to have a super strong stack kill dozens of units for no losses.
Anyways, if any of you need a break from the strain of SC or SC2, check this out:
http://www.warbarons.com/