I have seen a lot of questions on this site about Dwarf Fortress. What is it? What genre of game is it, and where can I get it?
Slaves to Armok II: Dwarf Fortress is a single player, tile based, real time strategy, city building, and adventure game by Bay 12 Games developer Tarn Adams. The game is playable on PC, under Windows (XP+), Mac (OSX), and Linux (Most distros, contains a text only mode). Most game content is procedurally generated and the game is highly moddable. One unique feature is the continuity of the world you play in. You can play many games of fortress and/or adventure mode in the same world, each building up it's history.
It features three modes:
So to answer your question about genre, it doesn't fit into any one genre.
C. Ross answers the "what", but a more important question might be "why". Why is this game so interesting? By modern gaming standards, it has a lot of strikes against it. By default it's completely non-graphical; it has an extremely steep learning curve; there's no game-defined goal or success state, even less so than other popular sandbox games; it can be really, really, hair-pullingly hard.
This is a game that isn't going to appeal to most people.
I'm speaking as a relative new-comer to DF, but some of the reasons this game is beloved by its followers are the exact same points I listed above. Its non-graphical nature (or lightly graphical if you choose to install one of the graphics packages) allows you to overlay your imagination on top of this world you're creating. The steep learning curve means there are always new challenges, new complexities in the game to explore and exploit. The lack of end-goal means you get to define your own experience. The difficulty keeps you from getting into that "gee, I've done everything, now I'm bored" state that you can find in other games.
So back when all of these virtues were not so obvious to me, what made me start playing Dwarf Fortress? I present to you the tale of Boatmurdered. If you can read through that and not laugh out loud, this game might not be for you. (But give it a try anyway! ;) )