Definition
Roguelike is a video game genre descended from the 1980 game Rogue, which was developed by students at the University of California, Berkeley—the genre takes its name from that game.
How it works
Commonly cited criteria for the “like Rogue” label include turn-based play, grid-based maps, procedurally generated levels and permadeath—when the character dies, progress is wiped and a new run begins from scratch.
Usage
Classics such as NetHack carried the tradition forward, and popular titles like Diablo drew inspiration from it; lighter variants that soften permadeath and other elements are known as roguelites.
Related and confusable terms
A mod is a player-made modification of an existing game, while roguelike is a game genre; one is about changing a game, the other about a category of games—they are not on the same dimension.