Pokémon World is the name given to the Earth-like planet (possibly a parallel universe version of Earth) that the Pokémon franchise takes place on. Humans live on this planet alongside Pokémon as a human-animal-like relationship.
There have been numerous fan theories about the existence of Pokémon World, and its connection to Earth. Although it is unlikely, it may be that humans have inhabited another planet somewhere else in the galaxy, in addition to Earth. Another suggestion put forward by fans is that the story may take place in a parallel universe that also includes humans.
It is also assumed that it could be set sometime in the future, mostly because of the fact that people use enhanced technology such as Poké Balls, teleportation (in the PCs), and mechas (Team Rocket).
Animals have been mentioned many times, such as in Pokémon Shipwreck, when Ash saw a fish when looking out the window. In one episode, a Pidgey could be seen eating a worm. Also, when Ash caught his Caterpie, Misty mentioned that she hated bugs; however she could be just referring to Bug-type Pokémon. But solid proof of common animals existing is the Pokédex entry for Gastly:
A being that exists as a thin gas. It can topple an Indian elephant by enveloping the prey in two seconds.
The entry for Raichu states in Chinese states that its electricity is so strong that it can kill an Indian elephant. However, this is not used in the English version of the entry.
Locations from Earth have been referenced, despite not appearing in the games, such as Mount Everest in one of Delibird's Pokédex entries, and Hollywood from Meowth's backstory in the anime. Arcanine's entry states that this is a Pokémon from China. Also, in the movie Mewtwo Strikes Back, Ash said Vikings mostly live in Minnesota (which references the Minnesota Vikings sports team). Also, there have been episodes that referenced Las Vegas and Chinese food.