All of the information from one record can be transferred to another record including addresses, phone numbers, gift records, memberships, and profile data.
Note that the record that is kept is considered the master record. No fields will be overwritten in the master record. However, if there are fields in the merged record which contain data and are blank in the master record, that data will populate into the blank fields.
For example, if the merged record contains a prefix but the master record does not, the prefix will be merged into the blank data field in the master record. So when the records are merged, the final record will now include the prefix.
Data like categories, comments, and custom fields will be added to the master record, but not replace any of the existing data. It is always good practice to review the final record after the merge has completed. You may need to correct things like the salutations, default addresses and emails, and some custom fields like custom true/false data.