GIS stands for Geographic Information Systems. GIS is an integrated collection of computer software and data used to view and manage information about geographic places, analyze spatial relationships, and model spatial processes. A GIS provides a framework for gathering and organizing spatial data and related information so that it can be displayed and analyzed.
To generalize, GIS is a database linked to a map. This means that any information you may have, which has locational data (Latitude, Longitude; Address;...), can be placed on a map and give you endless possibilities with which to use for analysis.