In the world of clique techtonics, there are three major groups: culture, counter, and dork. Culture houses jocks and preps, etc. Counter is for goth, punk, etc. Dork is where you get your geeks and nerds. Dorks lack the proper social skills to fit in with either the culture or the counter-culture. All geeks and nerds are dorks, but not all dorks fit into these categories. Geeks and nerds are generally notable for an interest in knowledge. Nerds those whose pursuits are solitary and generally academic in nature. Geeks tend to be into more trivial subjects and tend to congregate around them. Hence you have the "math nerd" and the "board game geek". But wait, wouldn't socialization take geeks out of the dork classification? Not really, because you don't have to be good at socializing to fit in with geeks. They are usually very accepting of a wide variety of social protocols. That's why the tattooed, foul-mouthed Warhammer geek and the bad-smelling Star Wars tie-wearing computer geek can both rock out to MC Frontalot and have a fun conversation about the merits of Tom Bombadil without totally getting on each other's nerves.
Technically this definition would make nerdcore hip-hop a geek thing and not a nerd thing. But "geekcore" sounds like a chicken-biting organization, and no one's into that.
PS - Yes I did make this up, but I did so more than a decade ago, which is like 80 internet-years. So it's basically a set of traditionally-held views at this point.