The Stand starts as a mistake is made in a US government biological warfare lab. A deadly virus (later nicknamed Captain Trips), which spreads at an alarming rate (99.5% contagious, I think) is accidentally released into the laboratory air. Although the lab closes, one man escapes, and, to put it briefly, contaminates the world with it. There are only a handful of survivors, all of whom start to have vivid dreams about one of two subjects - they either dream about an old black woman called Mother Abigail, or a man named Randall Flagg, also commonly known as the Dark Man, or the Walkin' Dude.
The two groups of people start to meet up, with all the social implications typical of post-apocalyptic books - people fall in love, people get jealous, people get very depressed... Anyway, Flagg's group set up in Vegas, and quickly get organised, due to Flagg's strict regime. The group under Mother Abigail's guidance reach Boulder, Colorado, and start life there. Flagg has many technicians and scientists, and so gets power on far quicker than the group in the Boulder Free Zone, as it is now known. Eventually, however, both groups form a basically stable town. However, Flagg is set on destroying the Boulder Free Zone.
I haven't really touched on Flagg yet, as he is a subject too big to deal with easily. He's certainly supernatural, and he has many personas, all of which begin with the initials RF. He turns up quite a lot in other books, under various guises. It is a subject of much debate about whether or not certain characters are related to Flagg or not. Almost certainly Richard Fannin from the Dark Tower series is another persona. Whenever I find any more, I'll put them here. Anyway, he is basically a force of chaos... I won't go any further in describing him, as I can't possibly do the topic justice. My advice is to read The Stand to find out about him...
The Boulder Free Zone send out a few spies to Vegas to see what is going on. After various interesting happenings between various opposition charcters, and people switching side, etc, one of Flagg's less sane followers decides to try to please him by bringing him a nuclear weapon with which to destroy Boulder. However, all goes horribly wrong somehow due to Flagg's fireball of wrath and the weapon explodes, destroying Vegas and all nearby - although Flagg disappears just before the bomb detonates. Is this the last of Flagg, I hear you cry? Well, not quite... the epilogue finds him in the jungle, where a previously uncontacted tribe look at him with curiosity...
This book is definitely worth the huge amounts of time you need to spend reading it... and if you can possibly get hold of the unabridged version, do. I haven't read the abridged version, but I know it misses out quite a lot - it wouldn't be the same mindbending experience without those extra few hundred pages.
Jon Skeet