Your contacts are divided into three different buckets: Prospects, Candidates and Clients. Your contact will move through a progression starting from a Prospect to a Candidate and then ending as a client. The status of the opportunity will determine which bucket the contact is in.
All contacts will start off as a prospect. A contact will remain a prospect if there is no known opportunity OR if there is an opportunity with a 'lead' status.
A prospect becomes a candidate if there is a quoted opportunity added.
A candidate becomes a client if there is a sold opportunity.