Mix of rogue-like, city-builder, tower defense and management training. Lots of features, but be prepared for text mode.
You control your dwarves by giving them tasks. Where to dig, what to produce, set training schedules for your military squads, and so on. If you designate a manager and give him an office, he can help you a bit. But don't expect wonders - he won't build dwarven industry, military forces or dwarven atom smasher - those are for you to be planned and done.
Be prepared for losing. Bigger population and more wealth attracts more aggressors. Digging too deep will have consequences, as well - there are much worse things than goblins out there. You've been warned.
For those who prefer classic rogue-like style, there is adventure mode. Bonus feature here is that you can come and visit fortress you have made in fortress mode.
If you like strategy, cRPG, challenges and don't mind text-mode, definitively worth trying.
If you are looking good introduction, you can try one at http://df.magmawiki.com/index.php/DF2010:Quickstart_guide - lots of useful tips there.
Game is still actively developed - new releases are still coming (v0.31.25 about 4 months ago), bringing more features every year.
If you prefer a bit better visualization than just pure ASCII, graphics sets are also available, you might try it out (I liked Ironhand's most, but YMMV).