If you've played a roguelike, you know what to expect. If you haven't, then Caverns of Underkeep is a fine place to start.
You'll battle skeletons, dragons, and the Dark Prince himself in this epic, randomly-generated quest. You've got several class and starting options, as well as complete control over your character's stats. I recommend pumping your Hit Points, and slowly gaining the Str and Dex to wear better gear as you level. This strategy proved effective for beating the game.
Don't expect moving graphics. Roguelikes are generally ASCII or tile-based; the levels are randomly generated dynamic puzzles. Action is turned based, so you can think about your moves. You will constantly have to balance risk versus reward versus savings--not spending gold will help your score, but spending gold will help you stay alive. All resources have been funneled into a really nice system reminiscent of Crawl. At times the randomly-generated areas take on a truly imaginative, organic feel of a realistic area.
The scope is huge. Levels are rather gigantic, and there are 7 to explore. You can beat it in about 3-4 hours on your first win--much shorter if you speedrun. Although it lacks the depth of NetHack or Crawl, it's easy enough that you can use this one as practice for the big league roguelikes.
Check it out and play Caverns of Underkeep now!