Since there will almost never be a 1 to 1 mapping between pixels in the texture map (texels) and pixels on the screen, we must filter the texture map when mapping it to the screen. The way in which we accomplish the filtering depends on whether we are forward mapping or inverse mapping.
Source : Teaching Texture Mapping Visually by Rosalee Wolfe / DePaul University / wolfe@cs.depaul.edu