one of my favorite sub-genres in science fiction.
Depending on exactly when and what kind of a change is postulated, and how carefully it's thought through, alternate history can be a delight.
Most of the people who write it tend to go with: what if the South and won the Civil War, or What if Hitler had won WWII. Although there are also lots and lots of others. I haven't yet seen an alternate history novel or story that goes with the What if Al Gore had become President in 2000, although there is one out there called Christian Nation by Frederic C. Rich in which John McCain wins the 2008 election, but dies a few months later, and Sarah Palin becomes President. So the extreme fundamentalist Christian right gets in charge, with the sorts of consequences you'd expect. It is plausible and scary.
A different alternate universe one is The Mirage by Matt Ruff. In this one, on November 8, 2001, Christian extremists hijack four planes in the Middle East. Two are crashed into the Tigris and Euphrates World Trade Center in Baghdad, one into the Arab Defense ?Ministry in Ryadh, and the 4th is crashed by its passengers in the desert. A decade later mysterious artifacts have begun showing up which seem to be from a world in which the United States, which in this world is a collection of backward Christian theocracies, is the only super power and a similar attack had occurred there. It's quite well done and I liked it.
Sometimes the concept is that people can somehow move from one parallel universe to another. The best example of that is Alternities by Michael Kube-McDowell. In that one a man stumbles across a portal to another universe, and it winds up being controlled by the repressive authoritarian government of that world. During the time frame of the novel, which is set in the 1970's, they only know of six other universes, all of which are sufficiently close to our own (and yet very different in many ways) that operatives from the home universe can travel and conduct business in the others. Very, very good.
Harry Turtledove is the master of alternate universe novels. He has several different series going on, so you might want to look him up also.
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