A square has two pairs of parallel lines. This makes it a parallelogram, which is a four-sided polygon with two pairs of parallel sides, each pair being equal in length. A square is a kind of parallelogram whose sides meet at 90 degree angles and are all equal in length.
Other examples of parallelograms are rectangles and rhombi. A rectangle has two pairs of parallel sides that meet at 90 degree angles, and a rhombus has two pairs of parallel sides that are equal in length. A square is a type of rectangle and rhombus since it meets both their criteria.