Nethack is of a genre of text based games, called roguelikes. Traditionally, the game is drawn as ASCII characters as a top-down view of a dungeon which you wander through, to get the Amulet of Yendor for your god.
Which would be easy, if everything wasn't randomized every time you played, and that there's no way to save and restore your game. When you die, your game's over. People play for years without ever ascending (that is, winning).
One can download the game (which is open source) and find more information at www.nethack.org.