LOCATION: This is the name of an available geographic location.
MAPSET: Every GRASS session runs under the name of a MAPSET. A MAPSET may be a geographical subset or as large as the parent LOCATION. Technically they are subdirectories under any location. In a networked environment with several users working within the same location, mapsets play a special role. Users may only select (and thus modify) a mapset that they own (i.e., have created). However, data in all mapsets for a given location can be read by anyone (unless prevented by UNIX file permissions). The "PERMANENT" mapset usually contains the read-only base maps like the elevation model, while the other locations are readable and writable by their owners. The "PERMANENT" mapset also contains some information about the location itself that is not found in other mapsets (projection info etc.), thus it must exist in every location.
DATABASE: This is the unix directory containing the geographic LOCATTONS (also called GISDBASE)
REGION: The REGION may be the entire area or subset of the chosen LOCATION.
WINDOW: see REGION
CELL DATA TYPE: a raster map from INTEGER type (whole numbers only)
FCELL DATA TYPE: a raster map from FLOAT type (4 bytes, 6 digits precision)
DCELL DATA TYPE: a raster map from DOUBLE type (8 bytes, 10 digits precision)
NULL: represents "no data" in raster maps, to be distinguished from 0 (zero) data value
INTEGER MAP: see CELL DATA TYPE
FLOAT MAP: see FCELL DATA TYPE
DOUBLE MAP: see DCELL DATA TYPE
CATEGORY NUMBER: A numerical value for a map feature (e.g. road number). Also sometimes called CATEGORY VALUE.
CATEGORY LABEL: A text attribute for a map feature (e.g. road name).
Both CATEGORY NUMBER and CATEGORY LABEL are related to the dig_cats/ file. The internal vector ID is different from this.
Inofficial proposal for 5.1 (to be discussed):
Since there is some confusion about the therm "category numbers" and
"category labels", there is the suggestion to change these two terms
to common GIS vocabulary for GRASS 5.1:
5.0 5.1
category number -> index # numeric
category label -> attribute # text