An apocalypse is defined as an event involving destruction or damage on a catastrophic scale. Such a broad definition definition can include a wide range of scenarios. A super volcano, the AI singularity or nuclear Armageddon. It is therefore useful to categorise these apocalypse in order to understand them a little better.
The problem is that there a number of possible ways to do so. Nick Bostrom in his paper Existential Risks laid out four classifications of existential risk. Bang, crunches, shrieks and whimpers. While both useful and very interesting these categories are a little too abstract.
Instead we are going to use the categories of human, nature, religion and space. Conceptually these catagories are easier to grasp.