As a user, I don't often ask questions (in fact, I believe I have only asked two questions in total across the entire network, excluding meta). However I do a lot of answering, flagging, commenting and of course, editing.
Because I like things to be consistent, intuitive, efficient and definite, I've developed a system of structure and keywords for my edit summaries.
Other discussions on Big Meta and per-site metas make it clear that the quality of edit summaries can be the difference between acceptance and rejection, or lead to grief when somebody hasn't explained their intent or actions quite so effectively.
Finally, if one writes a summary, they can confirm to themselves what they have done, and whether it's (all of) what they were aiming for.
How do you write the summary when you make an edit?
I will write an answer that describes the system and keywords I use. Other answers that extend this system, or better yet describe alternatives, will help people to understand what others are trying to say when they write an edit summary, as well as how they might write a better summary themselves.