379: technical writing
Because many messages cross a reader's desk (or computer screen) daily, the first thing she is likely to ask upon seeing one is "Should I read this?" The "purpose" section answers that question. A very direct opening like "The purpose of this memo is to ..." is perfectly acceptable. It may be addressed to me.
The summary is a brief recounting of the entire memo, including discussion/background, conclusions, and recommendation. Its placement as the second section allows a reader who does not need to know the details to stop reading.
The discussion/background describes the method by which the conclusion was reached. This section provides details of the subject and the justification for the conclusion. In almost all cases this is the longest section of the memo -- (eg., five paragraphs of an eight-paragraph memo, or two pages of a three-page memo). It is directed to those who need to know details -- laboratory assistants and clerical staff, for instance.
In the case of a progress report, this section will include estimated costs, possible personnel, available facilities and supplies. It may take a narrative form.
This makes some sort of conclusion from the issues and problems described in the "Background/Discussion" section. It also describes where you intend to go from here.