Pokemon was originally a Game Boy game in Japan, which proved so successful, it spawned numerous sequels, and even an anime cartoon. The game is like an RPG in many ways - you begin as a Pokemon trainer, and you are given a choice between three Pokemon - Bulbasaur, Charmander or Squirtle. From here on, you must defeat and capture other Pokemon, which you can use to help you in later battles.
There were originally 151 Pokemon in total (the 151st being Mew, who made his first appearance in "Pokemon: The Movie"), however the Game Boy games "Pokemon Gold" and "Pokemon Silver" introduced another 100 never-before-seen Pokemon - which were never in the anime series.
The games were cleverly designed - the original two (Pokemon Red and Pokemon Blue) - each only had 140 Pokemon, and the only way to get all 150 was to trade with other Pokemon owners. Doubtless this pissed off a lot of people, especially those of us running ROM dumps on Gameboy emulators.
The cartoon is along the same lines, following the adventures of Ash Ketchum, and his friends Misty and Brock. Along their journeys, they frequently encounter the bumbling villains Team Rocket, but always manage to save the day, despite the fact that Team Rocket have seemingly unlimited resources at their disposal.
The Pokemon themselves, "Pocket Monsters", are (mostly) cute little critters, each with its own unique skills. They seem to exist solely for battle, and after winning enough fights, they can evolve into larger, more powerful Pokemon - for example, Bulbasaur evolves into Ivysaur then Venusaur.
For some reason, Pokemon are capable of speech, however they can only say their own name. Pikachu, for example, is frequently heard uttering, "Pikapikachu! Pikaaaa!!" However, one Meowth learned to speak before joining Team Rocket, and (not counting Lapras and Mewtwo) is the only Pokemon known who can talk.
After the release of Pokemon Yellow, and with the translated Gold and Silver versions on their way, it doesn't look like the Pokemon phenominon is going to end anytime soon. Like it or not, Pokemon are here to stay.