Bitmap graphics are made up of colored pixels. Pixels are very small rectangles (usually square, although in some video applications they are wider than they are tall) of varying colors that once put together give you an image. Bitmap graphics are usually (but not always) photographic in nature, capable of subtle graduated tones - often in the range of millions of colors per image. The problem with bitmap graphics is that they don't enlarge well as Photoshop needs to guess what color the extra pixels should be - this can result is loss of definition and a dramatic lowering in quality, depending on how much you enlarge the image. Common file formats for bitmap image data include GIF, JPEG and PNG for Internet usage and TIFF for print usage.