I have been asked this recently and like always Wikipedia is the best source of information:

“The roguelike is a sub-genre of role-playing video games, characterized by randomization for replayability, permanent death, and turn-based movement. Most roguelikes feature ASCII graphics, with newer ones increasingly offering tile-based graphics. Games are typically dungeon crawls, with many monsters, items, and environmental features. Computer roguelikes usually employ the majority of the keyboard to facilitate interaction with items and the environment. The name of the genre comes from the 1980 game Rogue.”

Wikipedia 2012

A couple of other things I add:

-Derived from the Dungeons and Dragons role playing game

-Emphasis on “hack and slash” combat

-Single player

-Experience points are commonly used

-Fantasy genre is commonly used

Dom