A Contact is a person associated with an Account while a Lead is a prospective client with whom a salesperson is developing a relationship.
Salesforce has a specific workflow that governs the conversion of a Lead to a Contact. When this conversion occurs, Higher Logic Thrive Marketing Professional (Thrive Marketing Professional) captures the information and updates the subscriber record's ID to use the Contact ID rather than the Lead ID. Additionally, after a conversion, the converted Leads may remain in any Salesforce Lead Reports where they previously resided. However, they are always excluded during a sync with Thrive Marketing Professional and do not appear in Salesforce Lead target groups.
NOTE: When a Lead is converted to a Contact, Thrive Marketing Professional retains all mailing history.