Whenever we look around ourselves, we usually see solids.
We can easily draw figures like triangles, squares, rectangles, etc. in our notebooks. These figures are called plane figures.
When we combine two or more plane figures, it forms a solid figure.
Spheres, cubes and cuboids are examples of solid figures.
For example, a thick rectangular book. A single page of it can be considered as a plane figure and all the pages (plane figures) form a cuboid – the thick book.
A plane figure has only 2 dimensions – length and breadth; while a solid figure has 3 dimensions – length, breadth and height.
So, for a plane figure only its area and perimeter can be calculated; while for a solid figure, perimeter, area and volume can be calculated.
Surface Area of Cuboid = 2(lb + bh + hl)
where l, b and h are the 3 edges of a cuboid
Surface Area of Cube = 6a2
where ‘a’ is the edge of a cube
Reference : NCERT class 9th Mathematics.