In two-dimensional texture mapping, we have to decide how to paste the image on to an object. In other words, for each pixel in an object, we encounter the question, "Where do I have to look in the texture map to find the color?" To answer this question, we consider two things: map shape and map entity.
We’ll discuss map shapes first. For a map shape that’s planar, we take an ( x,y,z ) value from the object and throw away (project) one of the components, which leaves us with a two-dimensional (planar) coordinate. We use the planar coordinate to look up the color in the texture map.
Source : Teaching Texture Mapping Visually by Rosalee Wolfe / DePaul University / wolfe@cs.depaul.edu