A quadrilateral is a four-sided polygon. There are many different possible shapes and variations a quadrilateral could take, but there are six main types according to the BBC.
A quadrilateral with four sides that are equal in length and corners with 90 degree angles is a square. A rectangle has two sets of opposite sides with equal lengths and corners of 90 degrees. A rhombus has all sides of equal length, but the corners may not necessarily measure 90 degrees. A parallelogram has opposite sides of equal lengths, but the corners do not necessarily measure 90 degrees. A trapezoid has only one pair of parallel sides. Finally, a kite has two pairs of sides that are of equal length and one pair of diagonally opposite angles that are equal.