(Adapted from: http://www.businesswritingblog.com/business_writing/2013/05/write-better-executive-summaries.html#sthash.fYmpgBjM.dpuf)
An executive summary is a brief section at the beginning of a long report, article, recommendation, or proposal that summarizes the document. It is not background and not an introduction. People who read only the executive summary should get the essence of the document without fine details.
The executive summary for a business report is the written equivalent of what you would relate to the VP of your division while taking the elevator to the 30th floor or walking to the parking lot with him or her. It's the core of your document. As a 30-second or a one-minute version of the entire report, the executive summary should answer the reader's questions in brief. This summary is normally written last, when you are certain about the contents of the document.
For a business proposal or a recommendation, the summary might answer these questions:
What are common mistakes writers make in executive summaries?
4. Using different terms in the executive summary from those in the report. If the summary mentions findings, the report should include findings--not observations. If the summary cites results, the report should describe results--not outcomes.
5. Having a mismatch in content. Whatever the executive summary highlights must be included in the report. Likewise, the report should not contain major points that did not appear in the summary.
6. Including too little or too much in the executive summary. Executive summaries should cover only the essential findings, results, or recommendations.
7. Repeating the executive summary almost verbatim in the conclusion. If a report contains a conclusion, it should be a wrap-up that drives home the main points--not an executive summary that highlights them.