Interpolation is the process by which software attempts to increase the size of an image by guessing what colour and brightness are the pixels to be added in. It does this by looking at the pixels already in the image. Bilinear interpolation refers to the process of interpolation whereby the 4 adjacent pixels are used to determine what the pixels should be, and bicubic uses the 16 nearest pixels to determine the pixel...and if that is not enough, in CS2, there is bicubic sharper and bicubic smoother...