Originally Posted by Wikipedia, English Version

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. We've found that summaries often pique the interest of contributors with expertise in the area. This may not be as necessary for "minor changes", but "fixed spelling" would be nice even then.

In the case of a small addition to an article, it is highly recommended to copy the full text of this addition to the summary field, giving a maximum of information with a minimum of effort. Put ft in front, as an abbreviation of "full text." This way, readers of the summary will be unlikely to check the page itself as they already know the extent of the edit. These kinds of edits allow users to check Recent changes, Page history and User contributions very efficiently - this also reduces the load on the servers.

If the addition is more than 200 characters, so it does not fit fully in the edit summary box, you should write a short summary of the changes you have introduced into the article. For an addition of, say, 400 characters you can also save time by simply copying that into the summary field. The excess will fall off, and the first 200 characters will usually be acceptable as a crude "summary".

Unfortunately you can copy only one line of text from the edit box into the edit summary box. The contents of further lines can be pasted at the end of the line. Thus, for example, a bulleted "see also" list is cumbersome to put in the edit summary box. One possible workaround for a new list is putting the list on one line, separated by the asterisks for the bullets, copying it to the edit summary box, and then, in the main edit box, putting the new lines before the asterisks.

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.