Culture in Matthew Arnold's definition is the pursuit of human perfection.
Different cultures have different norms of human perfection, and pursue different things; this fits Spengler's (the real one's) idea that a culture has a core proposition. Within a civilization, high culture is a culture's full expression of itself; it seeks out and engages with the contradictory or self-defeating parts of its values, and tries to advance the culture's understanding of what perfection is and how it can be attained. (What the postmodernists call deconstruction, and Gardiner called "high parody," is a fundamental part of this process.) Low culture is mindless entertainment; it doesn't try to shape its consumers in any positive way, and certainly doesn't advance the society's understanding of its ideals. Middlebrow/bourgeois culture tries to be high culture and fails -- but typically fails intentionally. (To be at the forefront of understanding is prestigious but uncomfortable, and the middle class wants prestige but not discomfort.) Mass culture is typically low, since low culture sells, but can rise to middle or high. Low culture, and occasionally middlebrow or mass culture, can produce things that high culture finds interesting and takes up -- and high culture can similarly cross-fertilize downwards. See Popular Culture in Early Modern Europe for some very interesting discussion of this, which was more common then than it is today.
"Culture" historically means "high culture": the thrust towards self-improvement, plus the continued refinement of what exactly you're trying to self-improve towards. This sounds uniquely Western (especially modern Western -- "Faustian" in Spengler's terminology), but you see the same process in other parts of the world, especially in East Asia.
The catch is that you can't engage in high culture without access to high culture -- so perhaps it's unreasonable of me to condemn poor rural areas for not participating in it. Or perhaps not? It's not just mass culture that the Internet makes available.
Regardless, it's high culture -- or at least middlebrow culture, which is regrettably easy to mistake for it -- that reverend_gonzo was lamenting lack of access to.