Examples of two-dimensional objects

A two dimensional object has a length and a width. In comparison, a three dimensional object has a length, a width, and a height.

A square and a circle are two examples of two dimensional objects. When you draw them on paper, you are only drawing them in two dimensions. They do not come out of the paper, as they have no height.

If they did have height, they would be three dimensional and would respectively be called a cube and a cylinder.