The official Pokemon Pokedex is an encyclopedia and guidebook that contains information about all of the different types of Pokemon featured in the games and television show. Pokemon each have a unique number, which is how they are organized within the Pokedex.
The Pokedex is used within the Pokemon games, as well as in real life. It lists statistics and information about each Pokemon, including the region they are found, their evolution details, what skills they can learn and their height or weight. In the game, a Pokemon is added to the Pokedex when the player encounters it, and the objective is to fill the Pokedex with entries for all of the Pokemon.