The story is actually pretty complicated to spell out here, but if you're interested AlabamaPink (RIP) did a wonderful review over at Pajiba. The Basics: Susan Marley, once a pretty, normal girl, has withered away, both physically and mentally. She wanders the town daily (and through the dreams of the townfolk), never speaking. People say she's a witch. Her former lover, Paul Martin, failed-husband, high school teacher, and town drunk, tries to help Susan and unwittingly sets in motion events that lead to horrifying evil being released upon Bedford. Susan's sister, Liz, is the protagonist of the story, but it's hard to say if she's the main character. Liz and her boyfriend Bobby (the son of Bedford's only doctor) have dreams of moving out of Bedford and never returning, but Susan has other plans for them.
Really there are so many secondary and tertiary characters that it's impossible to even begin to list. And I can't tell my favorite parts of the story without giving away some of the plot. It's not the best horror novel I've read (that title still belongs to King's 'Salem's Lot) but it's definitely worth a read.