Contact represent the external people you are in contact with. These could be customers, leads, etc. Contacts can be associated with an Account if the underlying product supports it.

Fields represent the attributes defined for various entities in the underlying product. Depending on the underlying product, custom attributes can be defined by a User on various entities like Ticket, Contact, etc. is_user_defined attribute within Field can be used to differentiate between custom and system defined Fields.

Ticket Status represents the completion level of the Ticket. Some products provide customizing the Ticket Status.

Users represent the people using the underlying ticketing system. They are usually called agents, team members, admins, etc.