Today people refer to GIS as a computer tool and that is true too because a computer can keep track of such vast quantities of data which would be hard to do by hand. A GIS can be thought of as a digital method of creating overlays of information. The computer allows the user to create custom maps and perform analysis about the resources being displayed. The ability to keep all that information linked is what makes a GIS powerful. Building a GIS is similar to the process of typing all your parcel owner data into a computer.
Today getting the maps into the computer in a format acceptable to the computer is the hard part.
GIS is much more than just automated map making. Another aspect of a GIS is the ability to keep all the maps in the same scale and keep spatial relationships true. This is why we use mylar to keep maps from distorting the spatial relationships stored on them. A municipality must have maps of different scales for different purposes. That is necessary because not all maps are useful for all tasks.