A Seven-Day Roguelike (7DRL) is a roguelike created in seven days. That is, the author stopped writing code 168 hours after they started writing code. A 7DRL can be created at any time during the year. However, general agreement was reached that it would be fun to schedule a specific week for a 7DRL Challenge.
What is a roguelike game? The genre began with the game Rogue, which was created in the 1980s. According to Wikipedia, roguelikes are a sub-genre of role-playing video games, characterised by (1) randomisation (for replayability), (2) permanent death (once a player-character dies, the game cannot be restored at an earlier point), and (3) turn-based movement – although, of course, there are exceptions to each of these principles in various roguelikes.
For historical and practical reasons, many roguelikes depict the game world using ASCII “graphics,” although often newer roguelikes use graphical tiles. Roguelikes typically involve dungeon crawls, with many monsters, items, and environmental features. Brogue is a good example of a contemporary roguelike.