Stoner Rock is a loosely termed genre of music that has heavy guitar riffs that are somewhat repetitive and hypnotic. It's closest relatives would be blues, rock, hard rock and metal (of all types). The music is choppy, bassy and almost always distorted to some degree with different types of vocals, mostly male, which tend to blend and harmonize with the music. The guitars are often tuned to D or open D to get that low sound of metal-like distortion that the standard E tuning doesn't offer. The term "stoner rock" most likely comes from the perceived association of smoking marijuana and the fans of this music. However, smoking marijuana is not necessary to enjoy the music. These are some of the bands considered to be stoner rock: T-Rex, Kyuss, Monster Magnet, Wolfmother, Black Sabbath, Fu Manchu, QOTSA.
Stoner Rock is fun to play on the guitar because of its simplicity in basic form; playing it is as simple as tuning the low E of your guitar down to D, then play the low D, A and D strings (the top three) open. From there, you can hit the 3rd fret and noodle the 5th fret on the same three strings - start playing this, while allowing the open strings to resonate between strums on the 3rd and 5th frets - you're playing the beginnings of Stoner Rock!