Mix pokemon and a human female. Imagine the world of pokemon, but instead of pokemon you have girls with varying abilities and generally treated the same way pokemon are treated, with gratuitous sex as part of their training/taming.
Well it's a
bit more complicated than that.
The pokemon world doesn't have a massive civil rights movement pushing for pokemon rights the way pokegirls do. Nor does it have a backstory more than three hundred years long, or the legendary pokegirls, many of whom are quite hostile to mankind and are actively plotting against it.
You've still got tamers treating their pokegirls in ways that haven't been done before--in point of fact, almost all pokegirl stories are about these "new age tamers" who treat their girls as friends and partners instead of servants--but the Pokemon world doesn't have the Illuminati-esque SEELE and Gendo Giovanni trying to maintain the status quo, or Team Rocket's much more lethal cousin, Trauma Team, who'd rather kill their targets and
then steal their pokegirls.
And probably most importantly, there's very few limits to what you can make into a pokegirl. Sure, you've got new takes on the classics, but you've also got pokegirls based on anime archetypes. For instance, there's a whole bunch of pokegirls based on magical girls; the Cardcaptor line comes to mind, as does the Magic Knight evolution chain. There's the ghost-type Succubus (always fun to have in a harem ^_^), there's a group of dark types based on the Sailor Moon monsters--daimons are preferable to everything else in that chain--and all sorts of holy and unholy pokegirls.
Hell, there's even Evangelion, the Endtimes Legendary Pokegirl who'll destroy the world if she ever wakes up. Let's see Mewtwo match that.
I even made one myself for a Pokegirls/Sailor Moon fusion I wound up shelving back in January; Warvern, the Hidden Dragon Pokegirl, a fighting/dragon type I made for Makoto's form.