The US Military accidentally releases a "super-flu" virus into the world and it wipes out 99+ % of the population. It's mainly about 2 seperate societies that emerge out of the survivors--one group is drawn to Mother Abigail, a 104 year old woman who each person in the group has dreams/visions about. They form a "Free Zone" society in Boulder, CO and are essentially considered "good" (though aren't always...). The other group of people dream of Randall Flagg, who is kind of an anti-Christ figure, and they form a seperate society in Vegas. And essentially it becomes clear that the "good" society has to either destroy Flagg's society OR be destroyed by them.
Lots more to it than that, but it's a really hard novel to describe. LOL. If you read it, I highly recommend trying to find a copy of the original version to read first. The unabridged is really fascinating to read later, but IMO it's a little slower to get to the action and harder to get into. I almost gave up on it until someone handed me the original version to read. And then later, I went back and read the unabridged and it's really great added stuff so I'm glad I went back to try it again; it was just harder to get into the book at first (for me, anyway...)