Most graphics formats appear in two forms - vector or raster.
When the computer displays graphics on your monitor, they appear as a rectangular array of pixels or picture elements. This is the idea behind raster graphics. Shapes, textures etc. are thought of as configurations of pixels and these pixel configurations are kept in raster graphic file formats. So the information for each pixel in the graphic is stored directly in the file. Thus a description of the image is stored in the file, not the image itself.