The purpose of a memo (the shorthand for "memorandum") is to inform but it can occasionally include elements of persuasion or a call to action.
The format is typically used for short messaging relating to policies, procedures, or related business within an organization. It is often written as mass communication which broadcasts a message to an audience or group as opposed to one-on-one, interpersonal communications like emails. Some informal reports of several pages also employ a memo format.
Memos are distinguished by a header that includes DATE, TO, FROM, and SUBJECT lines.
Other lines such as CC and BCC may be added if necessary. CC stands for carbon copy (useful for a long list of recipients) and BCC stands for blind carbon copy (recipients cannot see other email addresses except for the sender).
Organization of memos will either follow a direct or indirect approach. However, most will follow a direct approach.