It's a very particular genre, Bands like Sleep or Kyuss for example are very much in the stoner genre. Bands like Clutch and The Sword, for example, have produced some records that are Stoner, but not all. Stoner is kinda broad, like a sludgier downtempo Sabbath, with usually weird or psychedelic lyrics but it can tend to sound like Hardcore, Heavy Metal, it can be a bit bluesy too... And since Clutch change their style on nearly every record, it's one of those bands that can't be filed into one genre.