"Social differences" are mostly in the forms of accident of birth or choice. Actually in general way we all experience social differences based on accident of birth, for example-male or female, tall and short.
At the siame time some social differences are based on our choices.
For example, some people are singer or atheists by choice. The examples of Belgium and Srilanka also show both regional and social differences which led to social divisions. Srilanka has shown the example of linguistic and religious differences. Hence social difference can take different forms in different societies.