Death
Death is a common ailment in Dungeon Crawl, and almost invariably lethal.
Like most roguelikes, death in Crawl is permanent - unless you are playing a Felid with extra lives, or are in explore mode. When your character dies, your save file is deleted and the character is moved to the morgue. A bones file is created for the floor (if eligible), and a player ghost may be generated in order to antagonize you in later games.
The only way to die is to drop below 1 HP from damage. Sources of damage include monsters, self-injury, and lingering effects like poison.
Here's an example of a quick death by monster damage...
The goblin hits you with a club. You die... 18 Bob the Skirmisher (level 21, -2/15 HPs) Began as a Human Fighter on Sept 14, 2007. Slain by a goblin ... wielding a +0 club (5 damage) ... on level 1 of the Dungeon.
...and here's one by falling down stairs.
In your confused state, you trip and fall down the stairs. Ouch! That really hurt! You die... 122 Len the Firebug (level 4, 0/20 HPs) Began as a Fire Elementalist on Oct 1, 2007. Died by falling of an stair ... on level 3 of the Dungeon.
History
Crawl historically had a few other ways of dying, like falling into lava. See the Instadeath article for more details.