If a new contact is added, an "Added" record will appear with the date, time, user, and source. If a contact that existed before the upgrade to Release 25 is changed, a "starting detail" record will appear to indicate this is the date changes started to be recorded for that contact, along with the field that was changed with the old value, new value, date, time, user, and source.
When using the Customer Contact Change Log Viewer by Customer, you can view all adds, changes, and deletions for that customer for all of its contacts. When using the Customer Contacts Change Log Viewer by Date, you can view change history for all Contacts across all customer/jobs within a specific date range. And you can limit your query to just the changes to a specific field by selecting that field in the Fields dropdown.