Polygons are closed, plane shapes with three or more straight sides. There are two main types of polygons: regular polygons which have all sides of equal length and all angles of equal measure, and irregular polygons which have at least one unequal side or angle. Common regular polygons include triangles, squares, pentagons, and hexagons, while irregular polygons can have any number of sides of varying lengths and angles. Polygons are further classified as convex if their interior angles form an outward curve or concave if their interior angles form an inward curve.