Every trainer who starts their journey must get their first, or starter Pokemon from the local professor. There are several different regions and therefore several professors.
In the Kanto region, trainers get their starter from Professor Oak. From him, they can choose a fire type named Charmander, a water type named Squirtle, or a grass type named Bulbasaur. Professor Oak studies the relationships between Pokemon and Humans, he also invented the Pokedex. Trainers starting their journey in the Johto region get their starter from Professor Elm. They can choose the grass type Pokemon Chikorita, the fire type Pokemon Cyndaquil, or the water type Pokemon Totodile.
Professor Elm studies Pokemon breeding patterns, which is sort of the opposite of what Professor Oak studies. Trainers who start their journey in the Hoenn region get their starter from Professor Birch. They can choose the water type Mudkip, the Fire type Torchic, or the grass type Treecko. Professor Birch studies are entirely different from Oak or Elm. Professor Birch studies Pokemon habitats. Trainers who start their journeys in the Sinnoh region get their starter from Professor Rowan. They can receive the grass type Pokemon Turtwig, the water type Pokemon Piplup, or the fire type Pokemon Chimchar.
Professor Rowan is generally considered to be wiser than Professor Oak, he studies evolution. In the newly discovered Unova region, trainers can get their start from Professor Araragi. They can get either Snivy, Ohawott, or Tepig depending on which type they want. Professor Araragi is the only female Pokemon Professor and studies the origins of Pokemon.
Each starter Pokemon, no matter the region consists of a fire, water and grass type. Trainers however can only choose one. The starters are considered to be super rare, trainers usually develop friendships with their Pokemon, but their starter is there first Pokemon and definitely means just a little bit more to them. The Pokemon Professors ask trainers to assist their research by completing the Pokedex. It may sound easy, but if you want to complete the entire Pokedex it takes commitment and dedication seeing there is over six hundred unique Pokemon species now!