GIS is about the link between geographical coordinates and informational data. Thus, managing this data is one of the most important aspects of GIS. In GRASS data is linked to geography via a combination of 'categories' and 'attributes'. GRASS 5.0 has a very simple approach to the management of data: "objects" in the map (cells, lines, areas, points) have a category number and these category numbers are linked to labels. Both category numbers and labels do not have to be unique. The meaning of these values and labels depends on the type of map.
The following chapters will, therefore, deal with these notions as pertaining, respectively to raster, vector and site maps. In the last chapter of this section, we will look at a way to combine GRASS with a database, such as Postgresql.