Business memos are usually internal, that is between employees within a business, company or organization. Memos use short sentences, less formal language, and bullet points to convey important information. Memos always contain a header that tells who it is from, who it is to, the date, and the subject.
On the other hand, business letters are usually between two businesses or a business and its clients. Because of this, business letters tend to be longer and use more formal language