The Pokédex (Japanese: ポケモン図鑑 illustrated Pokémon encyclopedia) is a digital encyclopedia created by Professor Oak as an invaluable tool to Trainers in the Pokémon world. It gives information about all Pokémon in the world that are contained in its database, although it differs in how it acquires and presents information over the different media. However, they are also only given to a few Trainers at a time, generally to the ones that are felt to have exceptional potential and skill. There are different types of Pokédexes and each Pokédex is special to a specific region. Pokédex entries typically describe a Pokémon in only two or three sentences. They may give background information on the habitat or activities of a Pokémon in the wild or other information on the Pokémon's history or anatomy. Pokédex entries also include height, weight, cry, footprint (prior to Generation VI), location, other forms, and a picture of the Pokémon.
In the Anime
In the anime the Pokedex acts more like an actual encyclopedia rather than the pokemon only being registered if caught. All the trainer has to do if they encounter a Pokemon is point their Pokedex at it. Once they do so, the Pokedex will read aloud a summary of said Pokemon. A Pokedex is given to trainers who start out their journey while picking a starter Pokemon along with five Pokeballs. A person may as well search for a pokemon and it has been suggested that a trainer can type in general details of a Pokemon to see what comes up, as seen when Ash and the gang saw a skinny, black figure and after Ash went on his Pokedex, and an Umbreon came up (Although it was a Tepig).