Quadrilaterals are four sided shapes that are closed. They are polygons. Quadrilaterals can be classified broadly in two ways. If the quadrilateral has a two pair of parallel sides then it falls in to the parallelogram category. Further now it can be broken down in to other shapes like square, rectangle, rhombus. If it has only one pair of parallel side then it falls into the trapezoid category. To compute the are of a trapezoid first you need to know the lengths of the parallel sides. Add the lengths of the sides and divide it by two. Now multiplying this average length of the sides with the height of the trapezoid gives the area of the trapezoid. To compute the area of a square or rectangle multiply the length of its base with its height.