You are correct: Cardhop, and Fantastical for that matter, are not "plug-ins". Cardhop is an alternate front end application for the Contacts database and Fantastical is an alternate front end application for the Calendar and Reminders databases and can show Weather data as well. In that way the connections between those databases and Mail, Messages, Phone, FaceTime, Maps remain unaffected and any change made in Cardhop appears in Contacts and vice-versa. On the other hand Cardhop can establish, recognize, and display relationship information that is not available in Contacts.
