There are five Regions.
For Pokemon Adventures, each region is explored by manga versions of their game counterparts. E.g. Red was the main character of the Red/Green/Blue chapters, with Green and Blue as his...sidekicks? Something like that.
After the Red/Green/Blue chapters end, Yellow becomes the main character. The story still takes place in Kanto, differs greatly from the storyline of the games and anime, and the main character is not Red/Green/Blue anymore.
For the Johto chapters, the main characters are Gold/Silver. And so on.
I believe (and am probably right) that Red's story involves Red/Green/Blue chapters, a bit of Yellow chapter, some small parts in Crystal chapter (maybe Gold/Silver too but I didn't read that), some small parts in the Emerald chapter, and the FireRed LeafGreen chapters.
And yes, simply put, each "chapter" (R/G/B then G/S and so on) stars different characters. Just imagine a manga version of the games, staring the player characters.
For further explanation, try bulbapedia. It's similar to wikipedia. Just search up stuff like "Pokemon Adventures", "Red", "Yellow", "Ruby", "Diamond" and so on to learn more, but I think my explanation would suffice.
You can read this to begin with, and use it to figure out how the Pokemon Adventures plot goes. Beware spoilers.