Below are a 5 resources that will help you quickly get to and understand GIS terms and definitions.
ESRI – Geographic Science, GIS Software Development, and Data Analytics
GIS Geography – A website about Geographic Information Systems (GIS) and Geography.
GIS Lounge – An information portal primarily relating to the field of geographic information systems.
Open GeoSpatial – A not for profit organization committed to making quality open standards for the global geospatial community.
GIS.com Wiki – This is basically a GIS Glossary. GIS.com forwards to ESRI.