Most of the 2-Dimensional shapes you'll come across in KS3 Maths will be polygons. A polygon is a 2-Dimensional plane (flat) shape with straight sides, such as a triangle or a pentagon. Polygons with equal angles and sides with equal length (like squares or equilateral triangles) are called regular polygons. All others are called irregular polygons.
Some polygons fit together side to side as tiles to cover an area without leaving gaps. This is called tesselation. Squares fit together very well, as do regular hexagons and equilateral triangles. Some shapes, however, leave gaps between their edges and just won't fit together.
There are certain rules which polygons follow. For example, the exterior angles of a regular polygon always add up to 360o, or the interior angles will add up to 180o x (number of sides - 2). A triangle has 3 sides so 180o x 1, a quadrilateral has 4 sides so 180o x 2 etc. Test your knowledge of polygons in this quiz.