Deactivating a user will cause all of their lead and customer records to have their owner be set to Unassigned.
Some other record types will retain the user's name in the Owner field until reassigned.
To simplify the transferral of records to a new user, we advise creating a work stack for each record type (e.g. customers, tasks), filtering on the Owner field, in order to show records currently assigned to that user.
You may then reassign all of these records before deactivating the user.