Executive Summary: Execs are busy people. Time is precious. State your main points in a few words up front. Perhaps they will read the body later, but more likely not.
An executive summary, then, is a precis, a synopsis of the content of a longer work. It surfaces the conclusions and recommendations as succinctly as possible, in a few sentences at most. This allows the reader to quickly evaluate whether they need to invest more time in reading the details, and if so, when. Like as not, the summary is enough, and the longer message can be archived for research, reference, or electronic search at a later time.
It differs from a lede. Unlike a lede, a well-written executive summary may actually discourage the reader from continuing. You've told them what they need to know, and if they trust you, that's enough. Why, then, even bother writing the rest? It's partly show your work—without backing matter, the conclusions of the executive summary carry less weight. It's partly because some analytical readers will read and value the detail that supports the conclusions. And it's partly CYA, because you may need to justify that set of conclusions later.
It differs from an abstract. An abstract is meant to help a reader decide if the document relevant to them, and thus worth reading. The executive shouldn't even see a document that's not relevant; it's a given that they are interested. The summary says "Here's what you need to know right now, the details follow for if and when you have more time."
It's an elevator pitch in the written word.
The reader leaves the elevator. The door shuts. Lord Brawl closes his jaw gently. A Muzak version of Careless Whisper begins to play. Screaming. Fade to black.