【Letter】 , 【epistle】 ,  【missive】 ,  【note】 ,  【message】 ,  【dispatch】 ,  【report】 ,  【memorandum】  are comparable when they mean a communication sent or transmitted as distinct from one conveyed directly from source to recipient (as by oral utterance).

【Letter】 is the ordinary term for a written, typed, or printed communication sent by one person or group to another most often by mail; the term carries no implications about the nature of the communication and no hint as to whether it deals with personal or business matters or with affairs of public concern.

【Epistle】 applies especially to a 【letter】 intended to be made public (as one of the scriptural letters of advice and counsel attributed to the Apostles) or to a composition in prose or poetry taking the form of an open 【letter】 . As applied to a private 【letter】 ,  【epistle】 is an overformal word typically used with some degree of humorous or ironic implication.

【Missive】 , too, is a somewhat formal term and as applied to a personal 【letter】 may be somewhat ironic or whimsical in implication.

【Note】 in general use applies to a 【letter】 that is brief and pointed, whether it is formal or informal. In diplomatic usage  【note】 is applied to a formal communication sent by one government to another.

【Message】 differs from the preceding terms in being applicable not only to a written, typed, or printed communication but to one that is orally transmitted (as over the telephone or by a messenger or servant) or is telegraphed, cabled, or radioed.

In official and especially governmental use  【message】 applies to a formal communication from the head of a state (as one sent by the president of the United States to Congress or by a governor to the law-making body of his state).

【Dispatch】  applies to a usually brief 【message】 that is sent posthaste (as by telegraph, cable, or radiotelegraph). In more technical use  【dispatch】  applies to such messages sent by an authorized correspondent to a newspaper or news association; it also specifically applies to an official 【message】 , often one in cipher, sent by or to a government to or by a diplomatic, military, or naval officer in its service.

【Report】  (see also ACCOUNT 2 ) applies particularly to a communication sent by an official (as a diplomat) to his own government. The term is also applicable to an official communication giving a detailed statement of facts, proceedings, or recommendations.

【Memorandum】  is used chiefly in business for an informal communication sent to an executive or employee, conveying instructions or directions.