I've seen "added 23 characters in body," "edited body," and "fixed capitalization in title." Which of these types of summaries (or another entirely) should we use?
You should describe what changes you made, and if necessary why you made them. Edit summaries like "added 23 characters in body" are automatically generated if you didn't provide one when editing. Sometimes this is OK if the edit is really simple or you don't have much time, but you really should always put something in there yourself when you can. It doesn't have to be long (or even a full sentence) but it should contain the essence of the changes you made. Like the edit on this post.
Admittedly, the majority of my edits have the automatically generated summaries, but that's because a lot of my edits are moderation-related. However, most users will edit to change the content, and in those cases the description is important.