In a short sense, memo is a written message from one person to another person within the same organization. Office memo is the short form of memorandum. The literal meaning of the word memorandum is a note to assist the memory. Memorandum is singular in number. Its plural forms are memorandums or memoranda.
According to Oxford Advance Learner's Dictionary, "A memorandum is a written note or communication specially in business between people working for the same organization."
A memo can be used:
To issue instructions to the staff.
To communicate regarding policy changes to the staff.
To give/seek suggestions
To request help or information to confirm a decision arrived at on the telephone, etc.
However, a memo may not be found appropriate if the matter is of a complex or serious nature involving lengthy discussion.
A memo can be filed for future reference. Therefore, it acts as an evidence to serve legal issues.