A good introductory definition for Service Catalogue inside Service Strategy ITIL v3 core book goes like this:
“The Service Catalogue is the subset of the Service Portfolio visible to customers. It consists of services presently active in the Service Operation phase and those approved to be readily offered to current or prospective customers. Items can enter the Service Catalogue only after due diligence has been performed on related costs and risks. Resources are engaged to fully support active services.”
The Service Catalogue is the tip of the iceberg from the Customers point of view.