Over the the Verso blog there are a couple of very useful posts about an ITSM topic that is often either neglected or misunderstood - the Service Catalog. The posts are What is a Service Catalog and Getting your Service Catalog started.
Serio has an excellent repository for Service Catalog information that allows you to log Incident, Problems and Changes against services from the Catalog, and to use that as the basis for reporting (for example Availability reporting). This repository is referred to as Systems.
You can find out how to create a System by using the Administrator HowTo guide (look under Configuration Management).