Most of these have really similar elements.
- A whole lot of people die, rapture or get kidnapped by aliens. In any case, they're gone.
- Loss of people results in loss of government, or at least a vastly weakened government. Survivors are suddenly on their own. This can be anywhere from one lone person who mysteriously survives a plague, to entire cities building walls to keep out the crazies in the wastelands.
- Frequently, though not always, the environment becomes hostile. This might due to radiation, flooding, ash clouds blotting out the sun, or zombies all over the place.
In future posts, I'll try to focus on some of these different kinds of apocalypses and try to bring out some of the differences.