With a paper map you see everything as a single entity containing all the information available unless a series of transparent overlays are included which modify the map for you. Until the arrival of computer mapping the only way to modify the base map was by marking the original, an expensive option, or by using plastic overlays which contained additional data making the map more meaningful for a specific purpose. For instance, crime statistics are often more easily understood when shown on a map than when provided in a spreadsheet or graph format. Prior to computerised alternatives, if this data was regularly updated, it would not have been economical to continuously replace the expensive base maps. Hence the tendency to use overlays.
Within a GIS, in addition to a base map, there are often many other layers of information. These are equivalent to the plastic overlays in paper maps and could represent various features such as rivers, lakes and towns for instance. Depending on your requirements, you can add as many layers as you require to make up your map.
Data is represented in your map in either a vector or raster format. Typical vector data would be used to represent rivers, lakes and towns. These would be shown in the GIS as lines, polygons and points. Raster data can represent elevations and depths. In the case of an ocean each dot within the raster could represent a depth. This raster data can be represented by a different colour to show different depths within the ocean layer. For instance lighter or darker blue depending on the water depth. Unlike vector data, which has related attribute data, the raster layer does not have additional attached attribute data. Within GIS vector data has both shape and size whilst raster data has numeric values rather than shapes. Again another way of explaining this to note that vector data is feature based whilst raster data is surfaced based.
Each feature on a map has a location. In order to correctly position points, lines and polygons within a GIS you need a coordinate system. This looks like a fine grid which is placed over the map. Each point on the grid is mapped with an x and y coordinate. Where a feature is positioned on the grid will be shown by its x and y coordinates.
If you would like to further explore how GIS could help your organisation jps services offers one and two day training courses in QGIS, ArcGIS and MapInfo.