Fun, quick and addictive. You always get that "one more game" feeling from it. I strongly recommend.
The point of the game is to sail around and take over the majority of the cities on a map. Each of these cities have different resources that once captured, generate those resources for you. Resources are then used to upgrade your ship, speed, amour or cannons or you can upgrade a city you own to make it harder for other teams to capture them.
The AI has a choice of 7 characters that all play differently. Some will chase you and attack, others will go hunting for crates and others will concentrate of getting cities.
I strongly recommend this game for the casual and core gamers. I've been playing for 20+ years and the complexity it brings while yet being very simple is refreshing.