What are raster maps

Raster maps are basically scans of a normal paper map. That means that the informations in the map are only available all together. In vector maps they are available in certain groups (streets, houses, landscape,…). That means that a raster map is just a picture in .bmp format. To make raster maps available for GPS devices they are being calibrated, which means that every point will be connected with the 'real' coordinates.
An advantage of raster maps in opposite to vector maps is, that they are usually a lot "prettier" and contain more information. The disadvantage is that you will always see all the information no matter how much you zoom in or out, which makes the map seem overloaded if you zoom out.

Here you see the same part of a map once as a raster map and once as a vector map.