Polygons are geometric shapes that have three or more straight sides and are closed figures. There are both regular polygons that have equal sides and angles, and irregular polygons that have unequal sides or angles. Common regular polygons include triangles, rectangles, pentagons, hexagons, and octagons, while irregular polygons can have any number of unequal sides and angles. Polygons can also be classified as convex if they curve outward or concave if they curve inward.