How would you explain the following? Draft, Letter, Memo. explain with examples.

Draft: Initial document created to be revised to make them more coherent or improve communication.  

e.g. First Draft, Second draft etc. of a book sent to editors as opposed to the final draft sent to publishers.

Letter: A document sent from one party to another to communicate on some topic.

e.g. Letter of Credit from bank to seller, Letter from Mother to Son.

Memo: A short message sent from one person (usually within the same organization)  to another.

e.g. An internal memo sent from HR to an employee