In the Pokemon DX series
The regions are structured similarly. Typically, a region will have its own Pokemon League, comprised of eight Gym Leaders. Once these eight are defeated, Trainers can challenge the regional Elite Four, which may or may not be shared with another, nearby region (so far, Johto do not have their own Elite Four; Johto shares one with Kanto). Each one also typically has a Pokemon Professor, who teach youngsters about Pokémon and give out Starter Pokemon to them. There is usually a villainous operating within the region, who uses Pokémon to further their goals.
Though there are many similarities between them, there are some major differences between the regions, such as what Pokémon are located there and what legends there are waiting to be uncovered. Visitors from another region quickly realize that the region they are visiting is far from their home, both in the way of the climate, and in the culture of the people there.
Professor Oak explains that Sinnoh, Kanto, Hoenn and the other regions are part of a larger region, most of which think this to be the Jetix region, but has yet to be proven.