What are Demographic categories?

One set of demographic categories is religious affiliation, which is whether a person is, say, Hindu, Christian, Muslim, Animist or of some other persuasion.

Another set of categories would be according to age: 0-5 years, 5-10, 10-20, 20-40, 40-65, 65 and older.

Each category is just one way of describing a human individual.