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.
Over the years, Ash Ketchum (Satoshi) has traveled to many different regions in the Pokemon world. Each new region brings a new generation of Pokemon with it and the anime features Ash and his pal Pikachu catching a new team of Pokemon with each new adventure.
In its entire run, the Pokemon anime series has never once shown a single clue as to who Ash's father really is. He is mentioned briefly as a 'Pokemon trainer on an adventure'. However, fans aren't quite satisfied with this theory, and have made theories of their own.