If you have experienced both GIS and CAD software you will probably find that most of what I have to say regarding similarities and differences familiar. A basic similarity between the two software facilities is that they both use a layer function to display the graphic element of the drawing (CAD) and map (GIS). Perhaps that is the most common similarity whilst most of the differences lie in the purpose each software type aims to fulfil.

Computer aided design systems as their name implies provide a means of designing objects. For the architect it will be some form of building design, for the electrical engineer it may involve a complex wiring diagram, for a civil engineer it could be a highway, tunnel or bridge project and for a mechanical engineer it could be some form of engine or pump design.  

There have been many definitions of a geographical information system which have successfully covered the elements of what such a system should be. However, for the purpose of this particular discussion we will narrow the definition to that of a computer based definition of a geographic information system. This is because there is significant evidence to show that GIS is now more or less associated with digitally based computer solutions rather than paper based solutions. Accordingly we could define GIS as a computer based tool which is able to convert geographic data into geographic information.