Geographic data can be used in a GIS in two different formats;
Raster data is simply an image of the map on the screen, similar to a digital photograph. Any of the maps you are familiar with in paper format can be viewed on screen as a raster file. All raster data is georeferenced. This means each file opens on screen in a position directly related to its position on the globe. In simpler terms, if you have two separate maps, one of Dublin and one of Wicklow, they will open on screen beside each other as you would expect.
Vector data is slightly more complex. All of the individual components of a standard paper map; roads, rivers, buildings, contours, towns etc are stored as separate layers. These can be viewed by themselves, or combined to produce a complete map.
Vector data files can be edited using the tools provided by the GIS, they can also be used for various calculations. For example, you can find out the length of a road or river, or the area of a town or county. This information can then be stored as an attribute of the file, so you only need to calculate the information once. As a result, each file has a table of information attached to it that you can use. More complicated analysis can be carried out on vector files using specific tools in the GIS software, however we will look at this in detail in the sections on software and in the case studies. All vector data is geo-referenced.