Investors, lenders, executives, managers, and other business leaders are extremely busy. I equate it to the saying:

They say I have A.D.H.D. they just don’t understand…oh look a squirrel!
That means that the Executive Summary has to be compelling and leave an unforgettable first impression for your business plan to get read.  Sometimes the Executive Summary is the only thing the decision maker will read.  Your goal is to get in the door and a great Executive Summary is the key factor.
What is an Executive Summary?
The Executive Summary tells the reader where your company is, where you want to take it, and why your business idea will be successful. If you are seeking financing, the executive summary is also your first opportunity to grab a potential investor’s interest. 
The Executive Summary should highlight the strengths of your overall plan and therefore be the last section you write. However, it usually appears first in your business plan document.
Business Plan for Existing Businesses
Company Information
This section should include information about your business, the names of the founders and their roles, how long you have been in business, type of work you do, partnerships, number of employees and business location(s).
Highlights
This section should contain financial or marketing highlights, projected sales and profits, unit sales, profitability and keys to success.  This is a great place to put a colorful graph or chart to show sales, gross margin and profits.  Remember to follow each chart with an explanation of the numbers.
Product/ServicesDescribe your products and/or services.
Financial Information
A list of your current investors and your information about the relationship with your bank and possible credit references and/or trade partners would be important to mention here.
Future Plans
Where do you want to take your business?  What is the plan for the next three years?
Business Plan for Investors
Company Information
This section should include information about your business, the names of the founders and their roles, partnerships, patents or technology, and special marketing approaches. 
Problem
You must identify the problem that the investor will solve.
Solution
How do your product and/or service solve the problem?
Target Market
Explain who your ideal customer is.  It is not everyone but a specific group of people or businesses that need your product and/or service.
Competition
Who is your competition and why are you better?
Goals
What are the upcoming goals and how do you intend to achieve them?
Guidelines
1.      Should be two pages or less in length.
2.      Written in language appropriate for the target audience.
3.      Consist of short and concise paragraphs.
4.      Have a conclusion.
5.      Should be able to stand alone if the rest of the business plan or report is absent.
6.      Make readers want to read the whole business plan.  Be compelling.
7.      Write it after you have written everything else in your business plan.  This way you can summarize and explain your company.

Often an investor or business leader will make a decision to continue reading a business plan with the impression the executive summary gives.
References:  http://en.wikipedia.org/wiki/Executive_summaryhttp://www.sba.gov/content/business-plan-executive-summary, http://articles.bplans.com/writing-an-executive-summary/