Losing is fun!
If you like city and empire builders and did not play DF yet your missing out on some of the best games you ever played. Literally anything is possible while building a fortress. The destiny of your dwarves is pretty much doomed but how long will they survive and thrive under you command?
Gameplay:
You start off building a new world where other civilizations have thrived and/or died. Then you will choose a site for your 7 dwarves to embark on. On embarking you get points to spent on skills for your dwarves and tools, materials, food, livestock, weapons, etc to take with you. Once you embarked you find yourself on a top down 2D map.
This map consists of a lot of layers, and you can get down or up a layer to view the surroundings on different layers. This feels very awkward when you first start out and it's hard to follow what's going on. But you will soon get very used to this and flow over your map like a real pro DFer.
So i hope you have picked a miner dwarf and brought him a pickaxe, since mining is what the game is about. Well, you could build everything from wood like the puny elves but everyone knows what dwarves do best. Now you can designate area's for your miners to dig out, put you livestock in a pasture, fish, hunt, etc. Once your dwarves have carved out some halls and rooms in the mountain you can start building workshops from the material they left. There are dozens of workshops but most importantly you need to build some furniture like beds to make bedrooms and tables/chairs for your dining room. Depending on how much food and booze you brought in you want to set up a food and booze chain up quickly too.
While you are designating this you will see your dwarves work on this, eat, drink, sleep, party or getting trampled by enraged elephants. Once you have the basics set up you are ready to make yourself a fortune, making crafts and statues from at least 200 different materials, inset gems into these objects, furniture, walls, weapons, etc.
Your dwarves get happy from nice rooms and seeing the wealth you got. Every dwarf has it's own personal likings, some like marble and some like onyx, some like salmon and some like goat cheese, some like beer and some like wine. You could use this to make them even more happy.
Each season migrants arrive, the amount depends on your wealth and how happy your dwarves are. This is the main way to grow your population. The other is children, your dwarves will get fond (or not so) of each other. And sometimes they make children too. When your population and wealth grows threats arrive, like goblin ambushes or even sieges.
This is where war comes in, you will have to defend your fortress from attacks. You can build walls, towers, moats, bridges, traps, balista's, catapults and an army. You will have to train your army and ballistic operators and make weapons and armor.
Then there is exploration, beneath your settlement are natural caverns filled with goodies but also creatures that are not so fond of your dwarves. You could get silk from giant spiders or capture. But you can also trap some giant flying creatures and train them to become mounts for your army.
The list of things you can do in dwarf fortress is endless. Believe me, i did not mentioned half of it. But it takes time to learn this and there are a lot of pitfalls along the way. One dwarf might go berserk and if he is strong and carries a legendary axe he could chop half your population to pieces.
Summery:
If you can step over the pretty steep learning curve and set aside it's awkward gui and low graphics then you will find yourself playing this a lot. It's great to see your dwarves do there jobs and make something magnificent. But it's also a lot of fun to see it's downfall.
Extra:
Dwarf fortress is originally ascii based like roguelikes. But there are plenty of tilesets and tools available to make your life easier. The lazy noob pack is a must have imo. Once installed you can install some graphics and put some preferences on or off which can not be done in game. It also has many tools once the game is running. The best one is dwarf therapist where you get a handy overview of all your dwarfs in a windows based raster. It shows the skills of every dwarf and you can turn jobs on or off.
You might also want to watch some video tutorials, there are plenty of them and they give you a sling shot over that steep learning curve.