The Custom Field information will take the 'master' records information - the record you are merging into the master record will have the custom field information superseded by that that is inside the 'master' record.
The History for the two contact records will merge together so that all history between the two records is combined into the one 'master' contact.
Please note you will also have a 'ContactMerge' history type which will give a summary of what has been merged into the record, as shown inside the history section below:
The Notes information - will take the 'master' records information - so the record you are merging into the master will have only the Notes from the master record and not the contact's notes you have merged into the master.
The Tasks however will combine together so that any tasks you had in the 'duplicate' contact will be merged into the 'master' record so you will see all tasks combined in this one record.