An edit summary is an explanation of the changes made to a given page in an edit. Although not required, editors are encouraged to write an accurate and detailed summaries (for more information, see Help:Edit summary on Wikipedia).
Properties
The edit summary box can hold one line of 200 characters. If you attempt to type or paste more, only the first 200 characters will be displayed - the rest will be disregarded. Also, for example attempting to add 10 new characters (at the end or in between) to a summary already containing 195 characters will result in the first 5 new characters being inserted and the second 5 being disregarded.
The "Show preview" button also provides a preview of the edit summary to facilitate checking links.
Recommendations
Always fill in the summary field. This is considered an important guideline. Even a short summary is better than no summary. An edit summary is even more important if you delete any text; otherwise, people may question your motives for the edit. Also, mentioning one change but not another one can be misleading to someone who finds the other one more important; add "and misc." to cover the other change(s).
Accurate summaries help people decide whether it is worthwhile for them to check a change. This may not be as necessary for "minor changes", but "fixed spelling" would be nice even then.
In addition to a summary of the change itself, the summary field may also contain an explanation of the change; note that if the reason for an edit is not clear, it is more likely to be reverted, especially in the case that some text is deleted. To give a longer explanation, use the Talk page and put in the edit summary "see Talk".
After saving the page, the summary can not be edited--another reason to avoid spelling errors.
In the case of important omissions or errors in the edit summary, you can make a dummy edit just to put the correction in the edit summary.
Places where the edit summary appears
The edit summary appears in white italics in the following places:
* Use the enhanced watchlist to see all recent changes in the watched pages, not just the last change in each page.
Section editing
When applying the section editing feature the section title is automatically inserted as initial version or first part of the edit summary. Put more details after this text. In the case that you provide a long summary yourself you can delete the section title in order to stay within the limit of 200 characters. The automatic text appears by default in gray, with the manually typed text being white. If you create a new section before or after an existing section by clicking a section "edit" link, please delete the automatic edit summary to avoid confusion.