There are 2 main types of data used within a Geographical Information System (GIS): vector and raster. Both models have a series of advantages and diadvantages when it comes to the storing of geographic information, and a fully functional GIS will handle both of them.
Raster data files consist of rows of uniform cells coded according to data values. An example would be a Digital Elevation Model (DEM), which represents ground elevations, in effect a continous layer.
Raster files can be manipulated quickly be a computer, but are generally less detailed than vector data. A drawback of the raster datatype is the disk space comsumed by the data, which is considerable more than a vecter approach.