A four-sided polygon is called a quadrilateral. "Quad" means four and "lateral" means side, so quadrilateral truly means four sides. The internal angles in a quadrilateral add up to 360 degrees.
Five common quadrilaterals have more specific properties: parallelograms, rectangles, squares, trapezoids and rhombuses. A trapezoid has one pair of opposite sides that are parallel. In a parallelogram, both sets of opposite sides are the same length and parallel to one another, while opposite angles are also equal. Rectangles are parallelograms where the angles all equal 90 degrees, and squares are rectangles with all sides of equal length. A rhombus is a parallelogram with four sides of equal length and is also called a diamond.