A roguelike game is a type of role-playing game named after the first of its kind, Rogue. Roguelikes have in common depth (you can do lots of things in them, though the level of detail varies from game to game), difficulty, lack of plot, and randomization. Basically, expect lots of tactical options and such, though Crawl has less than some others.
Roguelikes are not easy. Everything is a legitimate threat, to an extent, and this only abates slightly at higher levels. Things can kill you if you're not careful, even things you wouldn't normally consider dangerous. Nothing is pre-identified for you. And yes, you will often die in horrible, random, arbitrary ways. Oh, and if you die, the game erases your save file. Have fun starting over.
With a few exceptions (such as ADOM), a roguelike's plot consists of the following: go down into a randomly generated dungeon, reach the final level, and either kill the boss or, more commonly, grab an item and escape. Why? Well, in NetHack, you want to get this amulet for your god. In Crawl, you want the Orb of Zot, for some reason that's never explained. Xom could tell you, but then he'd have to murder you.
A good deal of what keeps the game entertaining. Dungeons are randomly generated every time, with only a few things you can count on. Your item selection, stats, gear, monsters you run into and so forth will be at least somewhat different every time, which makes the game addictive rather than repetetive and tiresome.
Dungeon Crawl compared to other roguelikes
Crawl seems easier, at first glance, but it's not. It is, however, friendlier. If something kills you, it'll probably be something you at least half-expected: an out-of-depth monster or a potion of poison at an early level, rather than a necklace you put on happening to be an amulet of strangulation.
Crawl also provides an in-game tutorial, manual, and more. The goal is that, unlike other roguelikes, Crawl should be playable without the aid of a guide or wiki. It also tries to remove no-brainers. For instance, in NetHack, you will be using, barring extraordinary circumstances, an amulet of reflection and grey dragon scale mail. In Crawl, you use whatever you find that happens to be useful in that particular situation, and many items or spells have drawbacks. Overall, the game is well-balanced.
Crawl sports some more minor design differences. There's less emphasis on class (Crawl's backgrounds), for one; your character is defined more by their skills, species and choice of deity. Each species also has different aptitudes for skills, and often suffer XP penalties for those advantages. Skills affect most aspects of your character's life and they are trained through practice.
Gods are one of Crawl's most interesting features. There are 17 gods in total, with each deity featuring its own credo. Once you've picked your god, if you act accordingly, you'll earn Piety. With enough Piety, your god may lavish you with abilities or various gifts to help you in your adventure. Of course, their code of conduct may be a pain to work with, and if you annoy them (by doing the wrong things, or converting) they will punish you, often very severely.