A parallelogram is usually defined as a quadrilateral with 2 pairs of equal, opposite and parallel sides. It can also form right angles between adjacent sides. Therefore, not all rectangles are squares, not all parallelograms are rectangles.
A parallelogram is a quadrilateral with two pairs of parallel sides, but there are no requirements for its interior angles. A rectangle, on the other hand, <em>must</em> have four interior 90 degree angles. So a rectangle is always a parallelogram, but a parallelogram is not always a rectangle.
