Pixelation: Pixels are tiny tiles that make up an image when arranged horizontally and vertically together on a a computer or LCD screen (which are originally captured as photosites by the image sensor in a SLR camera before being converted to pixels for display purposes). When you zoom right into a digital image, you will be able to see how each pixel contributes a single block of colour to the photo.
The number of pixels qualifies the resolution value of an image. The higher the number of pixels, the higher the resolution, and the clearer the image will be. When you enlarge an image past it's threshold on a device, you will notice pixelation.