A Geographic Information System is a database that relates any kind of information to places. Census and meteorological data are natural sorts of information to store in a GIS. With the right set of data, GIS can be an incredibly powerful tool for any kind of planning, from your daily routine to large-scale corporate or government projects. Like most powerful tools, it has tremendous promise in the right hands and terribly sinister implications in the wrong ones.
Examples of the sorts of questions GIS has answered in practice: