Raster (or image) files are pixel-based data, such as digital photographs, that you can use as backgrounds for feature and drawing data. For example, you can display an aerial photograph of a city under a layer of streets.
Image files can be very large, especially if they cover extensive areas in detail. To make them easier to handle, images are often broken down into “tiles,” which cover smaller areas. Each individual image file can be georeferenced, that is, it has an associated file (for example, TFW for TIFF files) that specifies the location of the image within the coordinate system used in the map.