While The End of the World as We Know It is very common in fiction, there are a wide variety of ways in which it can actually manifest; this can be measured on two sliding scales, depending on the size of the "world" in question, and to exactly what extent it "ends". This is derived in part from Bruce Sterling's analysis of ways in which the Real Life world could theoretically end.
Regional: a part of a continent or landmass, be it a state, country, or sub-continent (eg. "Sub-Saharan Africa", "California", "The British Isles", "Central America", etc).
Continental: an entire continent or landmass ("Eurasia", "Oceania", "The Americas", etc).
Planetary: an entire planet, or the vast majority of one. (If the given setting does not involve space travel and/or other worlds, then the scale effectively stops here, or skips up to Multiversal if the other worlds are not elsewhere in space, but do exist.)
Stellar: a solar system, every planet orbiting a star, the star itself, or the star plus everything in its orbit.
Galactic: a galaxy, most or all of its stars, up to all mass associated with it.
Universal: the entire universe, all or most galaxies within it, or all major galaxy filaments or equivalent highest-level structures.
Multiversal: multiple universes, or whatever that exists outside of the setting's native universe (includes whichever flavour of Another Dimension is on offer).
Omniversal: all universes or all possible universes, everything that exists, or reality itself; up to some abstract ontological limit if the setting includes explicit metaphysical stipulations.
Focused Destruction: A small localized area undergoes a species-level or higher apocalypse. The rest of the world at large is totally unaffected, maybe not even knowing of the events happening in the affected area.
Societal Disruption: Civilization survives intact, but is forever altered. This may be due to the sheer amount of damage caused lowering the standard of living, or it may be a result of people being forced to adapt to the new threat(s) they face.
Societal Collapse: Humanity backslides within the affected area, regressing to pre-industrial levelsnote most people are forced to be subsistence farmers at best and pre-agriculturalnote :most people are forced to be hunter-gatherers at worst. Civilization may recover on its own, but not for centuries at the least.
Species Extinction: A dominant or major species is either wiped out completely or reduced to such a low population level that its recovery is virtually impossible barring intervention by an outside force.
Total Extinction: Life itself ends. No living organism of any kind exists within the affected area.
Metaphysical Annihilation: The affected area ceases to exist totally, without remainder, or perhaps even to have ever existed; this usually involves erasing it from time. This may go up to the elimination of even the possibility of the existence of anything like the affected area, if for instance the basic system of reality is changed or wiped out. This may get highly abstract, depending on how fundamental the negation is.
Not to be confused with the comedy survival guide by Rob Kutner, which gives advice for surviving the relatively low-severity events. Or the Discovery Channel series by the same name which explores various scenarios in which this could play out.
Related Tropes (ways this can be brought about):