The primary goal in Pokémon is to find, capture and train various monsters, all in an effort to become the world's greatest Pokémon trainer. You'll choose one of three tamed "starter" monsters at the beginning of the game to help you capture other monsters.
There are 150 monsters in 15 different categories. Each category will be based on a specific characteristic or element, such as water, fire, flora, etc. There's a catch, though: there will be two versions of the game, a Red version and a Blue version, and not all of the monsters will be on any one Game Pak. To collect all 150 monsters, you'll have to trade.