@couchboy:
There are several differences between a business letter and a memorandum, most notably a memorandum is most often used for inter-office communications while a business letter is more formal and used for communications with persons and companies outside of the office. Business letters are also more often written in a standard format.
There are two general styles of business letters, a full block style where all elements are aligned to the left, and a modified block style where the elements such as return address are aligned down the center of the page. A business letter contains many elements such as the sender's name and address, the date the letter was written, a greeting, the main message, and a signature.
A memorandum may have any number of formats or the format may be specific to an individual institution. In the law profession, it is a record of the terms of a transaction or a contract. There are several other types of memorandums such as a policy briefing note, which informs employees of a change or an update to existing company policy. There are also bench memorandums, memorandums of agreement, a memorandum of association, and a private placement memorandum.
Texas CPA - licensed in 2012!!!