"Rogue-like" is a subgenre of hack-and-slash RPGs, codified by 1980 game Rogue.
- All levels and many other content are randomly-generated.
- No multiple loading of the same save file. You save when you leave the game and load when you enter the game. When you die, you must start from the very beginning.
- They are always turn-based.
- They use ASCII graphics.
- They are VERY hard.
- They have lots and lots of content.
- Player and computer play by the same rules, the only diggerence between player's character and NPCs is, well, the player's character is controlled by player.
- If you can perform some action, you can perform it anytime, there are no artificial limitations. If you can write in dust with your finger, you can do this from the very start of the game.
There is a subgenre - "coffeebreak roguelikes". They can have graphics and saves, they have much less content and are much easier for beginners. So you can play them to relax during the coffee-break, without being a hardcore gamer.
Another subgenre is "Rogielite", or "Rogielike-like". They also use procedural generation content and some other gameplay elements, but can belong to completely another genres, like top-down shooters.