Map projections
No maps of the earth are truly perfect representations; all maps have some distortion. The reason for this is because they are attempting to represent a three dimensional object (an ellipsoid: the earth) in two dimensions (a plane: the map itself).
A projection is a representation of the entire, or parts of a, surface of a three dimensional sphere on a two dimensional plane (or other type of geometry).
Why on earth are Projections used?
Every map has some sort of projection—it is an inherent attribute of maps. Imagine unpeeling an orange and then flattening the peel out. Some kind of distortion will occur, and if you try to fully fit the peel into a square or rectangle (like a flat, two dimensional map), you'd have a very hard time.
To get the peel to fit perfectly onto a flat square or rectangle, you could try to stretch out certain parts of the peel or cut some pieces of the peel off and rearrange them. The same sort of idea applies while trying to create a map.
There are literately...
