UV Mapping

UV mapping is the process of taking the faces of a 3D model and unwrapping them to create a 2D representation of the model. This is done because a 2D image of the 3D model is needed in order to make a texture. 
All game engines are only capable of rendering 2D polygons in a 3D plane. This means the information of all theses faces must be 2D as well. 

UV mapping appears to be very complicated but necessary. If a 3D object is unwrapped poorly, having a lot of distortion or having a bad layout means visual information can be lost or warped when a texture is applied to it. The concept makes sense to me and explains why some games have poor optimization in some areas possibly and stretched textures. In practice however this may be completely different.

http://cgi.tutsplus.com/articles/game-character-creation-series-kila-chapter-3-uv-mapping--cg-26754