A square is a parallelogram, but not every parallelogram is a square. A square meets all of the requirements to be a parallelogram, but it has additional properties that make it a square.
A parallelogram is named as such because it has two sets of parallel, equal sides that are each opposite of the other. When these two sets of sides join, they create two sets of opposite, equal angles. A square has all of the characteristics of a parallelogram, but each of the sides is equal in length and each of the four angles is 90 degrees. A square is also a rhombus and a rectangle.