Greetings!
Most of the important things have already been said. The complexity and flexibility of this game/toy is highly addictive and allows for various styles of play from murderous rampages of an adventurer out for genocide, over a cruel dictatorship fortress where all useless citizens are mercilessly killed to a peaceful trading fortress focusing on producing jewelry and the like for the various merchants visiting it. Even though still unfinished, it offers you incredible flexibility while the world itself is much more alive than anything you find in any commercial titles (check regularly the thoughts of your dwarves on their profiles page).
One piece of advice which has not been mentioned explicitly:
The Wiki is more or less the proper manual, so to speak. It contains a lot of detail information especially about how fortress mode works.
http://www.dwarffortresswiki.net/index.php/Main_PageAs for giving dwarves orders, you can only order soldiers around directly. For the other dwarves, you can only activate or deactivate labours in their unit screen ('v' then go over the unit 'p' -> 'l'). These labour then restrict the dwarf in what she will or won't do. Only a dwarf who has carpentry activated will go into your carpenter workshop and build a bed you ordered, for instance.
However, you can't tell a dwarf to do a task NOW. You can give out general orders to the fortress like "I want a carpenter's workshop built here", "I want this workshop to produce a bed", or "I want a tunnel dug out here", but you are completely at the mercy of the dwarves as far as the actual execution of the projects are concerned.
When playing fortress mode, you are usually confronted with four basic needs - food, drink, safety, and happyness. These are the things you may wish to look out for.
And DON'T build your first fortresses in haunted/sinister places or near rivers! I suggest building near a brook ... it makes life much easier (^_^;;
As for the question about roguelikes, actually only the adventure mode shows much resemblance to a roguelike.
A basic roguelike is a game where you control a single adventurer diving deeper and deeper into a randomly generated dungeon, slaying monsters and collecting treasures while at the same time facing the danger of starvation and dying of thirst. The name of the genre is derived from its first venerated example, namely "Rogue".
Roguelikes have quite a lot of freeware branches, like
Angband, and these branches usually keep the ASCII graphics that came with the original "Rogue". However, there are also a few commercial roguelikes.
The most famous roguelike is probably "Diablo". Besides adding graphics, it also moved into the realtime genre, but once you look beyond it, you can clearly see that it is just another roguelike. Other roguelikes I know are "Elderblaze" and "Masterblaze", both of which from the same company, adding graphics but keeping the turn-based designed of the original rogue and many of its freeware descendants.
The adventure mode in
DF can be considered a roguelike while the fortress mode actually moves into quite a different direction.
Deathworks