A quadrilateral is a four-sided polygon, which means it has four edges and four vertices (corners). The sum of the interior angles in a quadrilateral is always 360 degrees. Common types of quadrilaterals include rectangles, squares, trapezoids, and parallelograms, each with unique properties. Quadrilaterals can be classified based on their sides and angles. For example, a rectangle has opposite sides that are 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.