What is the difference between a 3-d shape and a 2-d shape?

3-D shapes  are 3 diminsions and 2-D is 2 diminsions

3-D shapes stand for Three Dimensional Shapes. Those are shapes with three dimensions. Some 3-D examples are cubes, pyramids, spheres and so on. 
2-D shapes stand for Two Dimensional figures. Those are shapes with only 2 dimensions, which make them flat. Some 2-D shapes are Triangles, circles squares and so on.

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