A site is a customer organisation containing clients, vacancies (and possibly candidates). Sites are viewed from the Sites form (use the View->Sites menu command).
A site may contain clients (listed in the Personnel list on the Sites form) but the site itself is not a client. A client is a person whose Client of list (on the Personnel form) has at least one consultant entry.
Sites are usually created for separate physical locations or organised along departmental lines. The division of companies into one or more sites will reflect recruitment and client policies.