Other forms: memos
A memo is a written message. Your boss may send around a memo in your office warning employees to not spend so much time socializing by the water cooler and to get back to work.
The intent of most memos is to help you remember something, or to remind another person of something. In a business or government setting, it's particularly important to keep written notes on decisions and communications between people. A written or emailed memo is one way to keep such a record. The word is short for memorandum, "thing to be recorded" in Latin, and a close linguistic relative of memory.