A diff is the difference between two versions of page. It can be viewed by clicking cur or last in the page history or on the Recent Changes page. The diff is also shown during an edit conflict so you can see exactly what you need to reintegrate.
How it looks 
The two versions are shown side-by-side. The old version is yellow and the new version is green. In left-to-right languages, the old version on the left. This is reversed in right-to-left languages. Text removed is shown in red on the old version. New text is shown in red on the new version.
The diff shows differences per line. Some editors find that adding manual line breaks improves the diff function.
As well as showing the difference between versions, the diff page has links to the user page and talk page of the user of edited both the last and current version. A link to the users' contributions lists are also shown. For sysops, a rollback button is shown allowing them to revert from the new version to the old one. This is only shown when viewing the diff between the recent version of a page and the last version by an author other than the one of the most current version.
edit summaries are also shown on the diff page. These appear in the row beneath the user names. If the user has used links in their edit summary, these act as links on the diff page as well.
This example shows the top of the diff page, with the links described above.
|Revision as of 22:32, Aug 03, 2003
When moving or copying a piece of text within a page or from another page, and also making other edits, it is useful to separate these edits. This way the diff function can be usefully applied for checking these other edits.
After the table of differences, the latest of the two compared versions is shown fully.
In the case of the Classic skin with quickbar, the diff page does not have the quickbar, to provide more space. Therefore the diff page is also useful for viewing the page on full screen width, without changing the preferences.
With the Monobook skin the panels on the left are also on the diff page.
Page widening is more likely on a diff page, because there are two columns, but also because URLs (long ones are often the cause of page widening) are not hidden.