An executive summary is an overview. The purpose of an executive summary is to summarize the key points of a document for its readers, saving them time and preparing them for the upcoming content.
Think of the executive summary as an advance organizer for the reader. Above all else, an executive summary has to be clear and concise.
In business, the most common use of an executive summary is as part of a business plan. The executive summary in a business plan serves the same purposes as an executive summary in any other document and one more in a business plan, an executive summary also has to entice the reader to read the rest of the business plan.
For this reason, the executive summary is often called the most important part of the business plan. If it doesnt do what its supposed to do, the business plan will be set aside unread.
For instructions and tips on writing an executive summary, see Writing the Executive Summary of the Business Plan, part of my Writing a Business Plan series. You may also want to read this Business Plan Executive Summary Example.