RPS is a game built upon gestures: a fist for rock, flat spread-out hand for paper, and the index and middle finger extended in a V-shape for scissors. Each gesture beats and is beaten by another. Rock crushes scissors but is covered by paper, paper covers rock but is cut by scissors and scissors cuts paper but is crushed by rock. As such, you always have a one in three chance of winning, based mostly on luck.
Both players face off and form a fist. Both shake their fists three times before making their gesture, also called a throw. Whoever wins the throw-down gets a point, then the next round begins in the same fashion. The first person to win three rounds is declared the winner, at which point you can start a fresh game. In instances where both players throw the same gesture, a stalemate is declared and no one gets a point.
Developed by Sam Kass and Karen Bryla and popularized on "The Big Bang Theory," Rock-Paper-Scissors-Spock-Lizard is a variant that adds two new gestures: "Star Trek's" Mr. Spock and a lizard. Spock is made using the Vulcan salute, with the index and middle finger split off from the ring finger and pinky, while lizard is thrown by making a reptilian hand puppet. The lizard beats Spock and paper while losing to rock and scissors, while Spock beats rock and scissors but loses to the lizard and paper. There are any number of alternative variants, mostly added in by the players for fun and featuring overpowered "beat-all" options like dynamite or shotgun.
RPS is played across the world under any number of names, sometimes with alternate rules but the same basic concept. The game features numerous variations and names in Asia, including Shoushiling and Huoquan in China and Sansukumi-ken and Jan-ken in Japan. Other variants include Ro-Sham-Bo, Hunter-Bear-Ninja and Ji-Gu-Pa. In all cases, while the weapons or gestures are different, the overall gameplay is identical -- each beats one but is beaten by another.