As a seminal piece of post-apocalyptica, Alas, Babylon can be best described as an example of dystopian fiction. A dystopia is the opposite of a utopia: a world in which everything that could go wrong has. It's like Murphy's Law made reality.
Alas, Babylon brings its readers' fears of nuclear war to life, showing the rampant death and destruction caused by one such world-ending conflict. Can't get more dystopian than that.
There's also a dash of old fashioned adventure fiction tossed throughout the novel. Whether we're talking about Dan Gunn's exploits as a doctor, Randy Bragg's takedown of the ruthless highwaymen, or even the group's constant struggles for food, our characters do all the same stuff you'd find in a classic western or pirate novel. Not only does this keep Alas, Babylon engaging and action-packed, but it emphasizes how the nuclear apocalypse has functionally sent its survivors back in time a hundred years or more.