The Pokémon Professor Organization (PPO) is an organization set up by the Pokémon Professors to promote Pokémon knowledge, helping young trainers on their journey and - of course - study Pokémon.
The members of the PPO are different Pokémon Professors. Each is from another region and has a different speciality field of study.
Professor Oak is often considered the most intelligent of all, and the highest authority about Pokémon. He is from the Kanto Region and specializes in relationships between Pokémon and humans. He is the inventor of the original Pokédex.
Professor Elm from the Johto Region was taught a lot of things by Professor Oak. He says he studies special Pokémon abilities, but, according to Oak, he is specialized in Pokémon Evolution. He is also an expert on breeding - it is said that he discovered Pokémon Eggs.
Professor Birch is a lot more active than the other Professors. In fact, he spends almost all of his researching outside in the Hoenn Region - he calls that fieldwork. His speciality is Pokémon Habitats and their distribution within them.
Like Professor Oak, he studies Pokémon Evolution, though the only difference is that he's much more active than the professor. He also has a specialty in Pokémon Evolution.
Professor Juniper studies where Pokémon came from, or in other terms, their origin. And in fact one of the first female professors to study Pokémon. Or to be the first professor to be a female. Her specialty is their origins.
He studies Mega Evolution and is trying to discover what is the cause to their Mega Evolution, currently. His specialty is Mega Evolution.