Understanding SharePoint's terminology is the first step towards unlocking its potential.

Site
A location in SharePoint where users can share data and information, view and edit web parts, collaborate, etc.
Site Collection
A set of sites that have the same administrative settings.  Each customer utilizing the Microsoft SharePoint service will have a single site collection.
Portal/Top Level Site
In the Enterprise environment it is referred to as the Customer Portal.  In the Foundation environment it is referred to as the Top Level Site.  The functionality differs between the two environments, but it always acts as the 'root' site of the site collection and the entry point into the SharePoint environment.
SharePoint Farm
A single or group of SharePoint servers that share a common configuration database.
Farm Administrator
The ITD representative(s) responsible for the configuration and administration of the entire SharePoint environment.
Site Collection Administrator
The customer representative(s) responsible for the administration of their portal/top level site as well as their site collection.  This person has full control of all sites within the site collection, can audit site content, receives all the administrative alerts, and is responsible for the initial site creation.  IT staff members are good candidates for this role.
Site Administrator
The customer representative(s) responsible for the administration of a particular site within the site collection.  This person has full control of their respective site and can create and administer sub-sites below it.  Divisional business power users are good candidates for this role. (Ex: the HR Director or their designee would act as the Site Administrator for the Human Resources Division site.)