In other words, a GIS combines layers of information about a place to give you a better understanding of that place.  What layers of information you combine depends on your purpose -  finding the best location for a new store, analyzing environmental damage, viewing similar crimes in a city to detect patterns, or many other possibilities.

What do you need to use GIS?  A full GIS requires:
Hardware
Software
Data
People
Training

For additional information about GIS, visit www.GIS.com.