Raster layers contain data a "coverage" of data that is sampled at equal intervals over an area. Often, these are satellite or orthophoto imagery (in which case, the reflected light from the surfaces is the data that was sampled), or LIDAR information (in which case it is the laser reflectivity that was sampled). In other cases, these data sets contain information from other sources, such as regular point samples of chemical concentration or regular samples of the height above a particular known level.
Styling the raster data was already covered in Styling Raster Layers.
Cartographica 1.5 and newer contain raster analysis tools that can be used to transform raster data.