A quadrilateral is a polygon that has four sides, four vertices, and four angles. The sum of the interior angles in a quadrilateral is always 360 degrees. Common types of quadrilaterals include squares, rectangles, trapezoids, and rhombuses, each with unique properties. Quadrilaterals can be classified based on their sides and angles. For example, a square has all sides equal and all angles measuring 90 degrees, while a trapezoid has at least one pair of parallel sides. Understanding quadrilaterals is essential in geometry and helps in various real-world applications, such as architecture and design.