A letter is a formal communication which has recently became less popular in favor of Email. Formal letters tend to be rare these days unless for legal purposes. Although banks and financial institutions usually use these.
The word Memo comes from memoranda - it was something used in business to record information which could be recalled at a later date, usually internally. Banking records about customers were recorded on Memos.
e.g. A bank might send an internal memo from their customer services dept to their billing dept reminding them when a bill is due for a customer, or perhaps to cancel billing. These were usually held in triplicate - with two more copies underneath the original copied using carbon paper. Hence the term CC (carbon copy) and BCC (blind carbon copy). One kept on the customer's record and the other sent off, the third passed to any other relevant person.
As everything is electronic now, any form of communication is usually performed using Intranet systems (rather than Internet) to send Internal Emails. Copies are held in storage files on Internal folders on servers, removing the need for any paper copies. It is still interesting however that the terms "BCC" and CC" has survived the transition.
Letters are completely external communications between companies and their customers. But even these are quite rare now.
I hope that helps.