Lots and lots of orcs. The players' goal is to slay as many orcs as possible while looting every treasure chest that they can find. Standing between them and their goal, however, is the hackmaster. The DM controls the orcs, and wages total war on his players. To win, he either needs to slay all the players, or keep them from winning for two hours.
Players acquire treasure cards by opening treasure chests.
These cards can be a mixed blessing. The Player cards offer special abilities that let them bring other players back from the dead, acquire henchmen, or otherwise make their lives easier. Like wise, the Hackmaster cards help the GM, giving him special powers that cancel out player cards, summon huge amounts of orcs, and generally make the players like more difficult. Many of the guys in my group are fans, so when I went to GenCon in 2000, one of the first games I bought was Orcs at the Gate. It took us a few months to find a free Friday to play the game, but when we finally did play it, we found that it was a great game marred by so-so execution.