Some examples of two dimensional shapes include squares, triangles, circles and rectangles. A two dimensional shape can be measured by the two dimensions of length and width but not by its depth.
The closest one can come to actually seeing a two dimensional shape is by looking at a paper cut out of one. The paper has thickness, however, so even these shapes are technically three dimensional. Three-dimensional shapes, such as cubes, pyramids and spheres, can be measured in the three dimensions of length, width and depth.
Some two dimensional shapes are known as polygons. A polygon is a shape with three or more straight sides that is closed. Triangles, squares and pentagons are classified as polygons. A polygon cannot have rounded sides, like a square, or intersecting sides, like an hourglass shape. In some instances, however, a circle is considered to be polygon that has an infinite number of sides. A polygon that has equal measurements for all of its sides and angles is known as a regular polygon. Squares and equilateral triangles are examples of regular polygons.
In addition to measuring the sides and angles of two-dimensional objects, its also possible to measure the shapes' perimeter and area. The perimeter is how long all the sides of the shape are when added together. The area represents how much space is inside the shape.