An Executive Summary is a commonplace component of lengthy documents, particularly reports. It is a section that appears at the beginning of the document.
The purpose of the Executive Summary is to provide the reader with a relatively quick overview of the most important aspects of the document. Ideally, the reader is able to read an Executive Summary in 2-3 minutes.
In cases where a document may be 20 pages or more, many readers may just read the Executive summary and not the whole document, unless they have a particular need. For this reason, the Executive Summary must be written well on the assumption that many readers may not go further.
Here is a definition of an Executive Summary.
The Executive Summary is a concise presentation of a report’s major findings, conclusions, and recommendations. It is a précis of the more extensively developed substance of the accompanying report. In both content and structure it is tailored to make an impact on the expected readers or readers of the report. (James C.Morrison, Massachusetts Institute of Technology)
When you are writing an Executive Summary you should make sure that it is no longer than 1/10th of the full report. The main points that you summarise in the Executive Summary should be in the same order as in the full report and each of the main points need only be written in 1 - 2 sentences.
It may be beneficial to readers with less technical knowledge if the language of the Executive Summary is simpler than the rest of the report. Try to avoid the use of jargon.
It goes without saying that the Executive Summary, as a very important section of your document, should be carefully proofread to remove errors and maximise readability.