Raster graphics come compressed and uncompressed. Uncompressed raster files, such as Windows bitmaps (.bmp) or X-windows pixmaps (.xpm) are very large, but they display very quickly, with little computation. This makes them good for such things as backgrounds.
In a bitmap a great many pixels are probably of the same color. Instead of listing the definition of that color 4000 times inside the image file, the computer can call that color ``2'', list its definition once at the begining of the file, and thereafter refer to it as ``2.'' This is the basic idea behind graphics file compression. By using this simple technique file size can be greatly reduced, and only minimal computation is required to display the file. Examples of this ``loss-less'' compression are GIF's (.gif) and TIF's (.tif).
The most effcient way of storing a raster file is a complex technique known as ``lossy'' compression, because some of the detail in the image is lost during compression. In theory (it usually works this way) lossy compression only removes details to small for the human eye to precieve. In practice lossy compression is great for photographs, but not for line drawings. It is also best used for images which will only be viewed on a computer, and not for images that will be professionally reproduced. The best known lossy graphics format is JPEG (.jpg/.jpe/.jpeg) which is used for many graphics on the Web.