All of the other record's gifts, receipts, recurring plans, timeline activities, tasks, event registrations, form submissions and tags move onto the record you keep, and lifetime giving is recalculated. Issued receipts move across for lookup but are never rewritten. Each keeps its number, amount and the name it was issued under.
Details
The record you don't keep is archived, not deleted, and old links to it open the kept record. Where the two records disagree on a detail, such as email or address, you choose which value stays during the merge. The merge is logged on the kept record's timeline.