It's a subgenre of games that has some combination of roleplaying, ASCII graphics (or low resolution bitmaps without animation), procedural content generation and perma-death. Most well known are NetHack and Dwarf Fortress.

People tend to enjoy them for two very different reasons. First is that focus on things other than graphics tends to improve depth and re-playability. Second is that without graphics, the player's imagination plays large part in building world. It's bit more like reading a book than watching a movie.