On a very basic level it a map. However, you can then start to show more than one layer of information. For example, you can show war boundaries over a backdrop map to see which addresses are in the area. This is called a spatial query, using a geography to give you an answer to a question.
You can also bring in your own information, for example, if you have a spreadsheet that has addresses or postcodes in it you can use an addressing product and within GIS mast the postcode in the spreadsheet to postcode in this product.
You can then see where all your addresses are and this allows you to see patterns such as clusters or gaps.
You can use a map to capture and create data from for example school areas or bus routes. Rather than a list of roads, the area can be drawn as a polygon and instead of a list of bus stops route can be drawn as a line. You can put buffers around features such as clearance around wind turbine or railway lines.
Once you have data in GIS you can start to look at things spatially, such as what is the distance between A and B, what is the area of something, what the length of something is or what is the distribution of something. This can be extended to query such as what points of the polygon are within the area or what features such as buildings are below a certain size.
If you have information that relates to areas then you can displays these as well. For example, with the general election, there is always a map showing areas of the country colored to match the party that has won. This is called thematic map, where the map is colored according to a theme, in this case, the winning party. However, this could also be population, unemployment, crime or anything that can be linked to a geographic area.
If you have a road network product then you can plot the rout between two points. You can then create an area of distance along the roads from the point, 10km, 20km or if you have information about speed you can do these in term of time. As you can see analysis and potential of geographical data is endless.