The document defines and classifies polygons based on the number of sides. A polygon is a closed plane shape with three or more straight sides. Polygons are classified based on their number of sides, with specific names given to polygons with 3 to 12 sides, such as triangle, quadrilateral, pentagon, etc. Polygons can also be categorized as convex or concave based on whether the vertices point outward or inward. Regular polygons are those where all sides have the same length.