Even though memos and letters are both forms of communication within business settings, they differ mainly in terms of their function and structure. First and foremost, memos are used to convey messages within an organization, unlike letters which are used to communicate with other organizations. For example, if a human resource manager wants to convey a message to all employees in an organization, a memo is most suitable. In terms of structure, memos have a clearly defined structure with headings and bullet points, unlike letters which are written continuously in paragraph form. In addition to that, letters could be as long as needed but memos are generally kept short and to the point. Whereas it is mandatory for formal letters to begin with salutations, this is not the case for memos which start immediately with the subject and end with the work objective to be undertaken.
A memorandum or memo is a means of business communication within an organization, unlike a letter which is used for communication between two or more different organizations or between a business and its clients.
A memo is used to convey a message when word-of-mouth is not sufficient. It is often prepared somewhat informally as compared to a letter, which is generally very formal. Memos have less attention to structure, grammar, formatting and quality of paper. Letters, on the other hand, are written formally, proof-read to ensure accuracy and avoid even the smallest mistakes.
A letter can be as long as needed, whereas a memo is generally kept short and to the point.
A memo generally has a header that states where it is from and the intended recipient, along with the date and subject. It does not need to be on company stationary. A letter is always written/printed on business stationary and includes the date, the recipient's name, designation/title, his/her address, and starts with an appropriate salutation.
A memo does not, generally, conclude or summarize anything and finishes with a work action (a to do activity). A letter summarizes or concludes the communication, asks the recipient to contact the writer and ends with thank you to the reader. The sender's name and address are also included.
Hope this helps.
A memorandum (often called a memo in the United States at least) is a form of business communication. It is typically used within a business as opposed to between two firms.
A memo is not generally written in a style that is as formal as a letter. This is, however, often not true if the writer of the memo is addressing the memo to a superior within the company.
A memo does not have the sort of greeting and closing formulas that a letter has. It does not start out "Dear..." or close with "Sincerely yours" or anything like that. Instead, it is often printed on a premade template with lines for "To" and "From" and "Regarding."
So I would say a memo is different in format and in the fact that it is used only within a given firm.