What is the difference between a proper and an improper subset?

A proper subset describes the set in full.  An improper subset strays from the original set.

An improper subset is a subset containing every element of the original set. A proper subset contains some but not all of the elements of the original set. For example, consider a set {1,2,3,4,5,6}. Then {1,2,4} and {1} are the proper subset while {1,2,3,4,5} is an improper subset.