I could go on and on, and there are many influential foreign films that can be added to this list, but Wikipedia is a good place to learn more.

Can you break down the different genres? What are the main differences (if any) between mainstream and B-horror flicks? Is horror global, so to speak?

How many years do you have? Really! Sub-genres equate to variations on a theme. And in horror, you have lots of variations on a theme. For simplicity, I look at it this way: you can break down horror films into supernatural horror, man-made horror, nature-made horror, slasher-psycho-killing-machine horror, creepy freaky aliens from space horror, and splatter-gore horror. Vampires, ghosts, demons, Lovecraftian alien-gods, etc, fit into the supernatural category, though with Lovecraft you can use creepy freaky aliens, too.

Man-made horror encompasses most of the 1950s mutant horrors like giant bugs, and even Frankenstein. You have your mad doctors, mad scientists in the man-made horror category, too. Right now, the slasher-psycho-killing-machine and splatter-gore horror films are in vogue. Unfortunately - I say unfortunately not because they aren't valid horror forms - mostly because the focus is on visually revolting and disgusting effects, not the story.

Zombies fall into the nature-made and supernatural horror categories, though many horror heads would label zombies as a sub-genre, too. I'm not saying you can't do that, but I like to be as broad as possible first, and then narrow down from there. Of course, many horror films overlap in their use of themes or sub-genres. The sky's the limit here. For a great tutorial on horror genres, I recommend reading Tim Dirks' article at Filmsite.org. He touches on the various genres that evolved at various times in the horror time-line, and mentions the associated movies for that genre and period. Very informative reading.

It's funny, but foreign horror films, like the Korean and Japanese films, take horror seriously. It's not an A or B or C movie labeling for them. Their actors and directors take what they do very seriously. It's mostly in America that this odd system of B and C movie labeling has evolved.

When you look at the early Universal films, especially Lon Chaney's silents, they were extravagant, main-studio productions. They kept Universal afloat during the Great Depression. As you move more into the 1940s, you start seeing lower-budgeted films specifically targeted for fast production. The stories were treated with less importance, and the studios started lining up lower-status actors to star in them. So the difference between a mainstream and B-movie then was the cost, story quality, production quality, and acting roster. But the funny thing is that they were made quickly and cheaply to turn a fast profit — and they made money!