Pay Day

Pay Day is a move that was introduced in Pokémon Red and Blue, and has been in all games since. Pay Day is a Normal-type move with a base power of 40. Over all of the games, only a handful of Pokémon are able to learn Pay Day, however.

Red, Blue, Yellow, Gold, Silver, and Crystal

In these games, when you use Pay Day in a battle and win the battle, you will obtain money equal to double your Pokémon's current level. So if you had a level 50 Meowth that used Pay Day, you'd get 100 Poké Dollars (the money in Pokémon games). This is a useful technique because in Red, Blue, and Yellow, the only other way to get money after beating all the trainers in the game is rebattling the Elite Four, which can be hard and time consuming. With Pay Day, you can get money anywhere.
Meowth learns Pay Day at level 17 in Red, Blue, and Yellow, and at level 20 in Gold, Silver, and Crystal. In Gold, Silver, and Crystal, you can breed Pay Day onto Hoppip with a male Pikachu or Raichu that knows Pay Day. More info on the Breeding page. In Red, Blue, and Yellow, TM16 is Pay Day.

Ruby, Sapphire, Emerald, FireRed, LeafGreen, Diamond, Pearl, Platinum, HeartGold, SoulSilver, Black, White, Black 2, White 2, X, and Y

That's a lot of games; all the games from Generations III (3), IV (4), V (5), and VI (6). In these games, the only difference is that the money gained will be five times the amount of your Pokémon's level. Also, if your Pokémon is holding an Amulet Coin while using Pay Day, your money obtained will double. So a level 50 Meowth using Pay Day with an Amulet Coin held will get 500 Poké Dollars. Without the Amulet Coin, it will get 250 Poké Dollars. From Black and White onwards, having your Pokémon hold a Luck Incense also doubles the money you gain.
Ruby, Sapphire, and Emerald Meowths and Persians can learn Pay Day at level 20, while FireRed and LeafGreen Meowths and Persians get an earlier level 18. In Generations IV (4), V (5), and VI (6), only Meowth learns Pay Day at level 30, so if you want Pay Day on your Meowth, wait two levels to evolve it. In Generations V (5) and VI (6), you can get Pay Day onto a Purrloin by breeding if the father is a Meowth that has Pay Day.

That's about all there is to Pay Day! Hope you learned how to get more money from your Pokémon.