Yukinari Ookido (Professor Samuel Oak in the English and Italian version, Professor Carvalho in the Brazilian version), is a Pokémon researcher, and generally considered the best of his kind. As such, his role in the Pokémon games and anime is that of a mentor to young Pokémon Trainers, a source of information and an occasional plot device. Amongst other things, he is credited with inventing the Pokédex. Although he is an expert on all matters relating to Pokémon, Professor Oak specializes in Pokémon behavioural science.
In the Pokemon 4ever movie, it is revealed that, as a kid, Samuel met a Celebi and traveled through time with him. He then came to the present time and became Ash's friend. After the defeat of Viscious, he travels back to his time.
A digital encyclopedia that catalogs the many hundreds of species of Pokemon, a Pokedex is a handy tool and one of the best friends for any Pokemon Trainer. While Pokedexes do function a little differently throughout the Pokemon franchise, their base functions remain the same.