Help:Diff
A diff shows the differences between two versions of a page on Wikipedia. A version of a page is created every time someone clicks on Save page after changing it. This version is called "revision". Each revision has a time and date when it was made. This is written in the history of the page. A diff lets you see what changed between two revisions of the same page.
There is a link to see the diff for every change in the page's history, in the user's changes, in new changes and in watchlists.
Example
This is how a diff looks like. A diff is useful because it makes it easier to see what changes have been made. It also allows to undo the change, or click on various links, for example to put a message in the talk page of the user who made the change.
Line 8: Line 8: style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%"| For sysops and those with the rollback flag, a [[en:Wikipedia:revert|rollback]] button is shown allowing them to revert from the new version to the old one. style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%"| For sysops and those with the rollback flag, a [[en:Wikipedia:revert|rollback]] button is shown allowing them to revert from the new version to the old one. − style="vertical-align: top; padding: 0.33em 0.66em; border-color: #FFE49C; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" | This is only shown when viewing the diff between the current version and the one immediately preceding it. + style="vertical-align: top; padding: 0.33em 0.66em; border-color: #A3D3FF; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" | 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. style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%"|This example shows the top of the diff page, with links the described above. style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%"|This example shows the top of the diff page, with links the described above. Line 25: Line 25: style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" | </table> style="vertical-align: top; padding: 0.33em 0.66em; color: #333333; background-color: #F3F3F3; border-color: #E6E6E6; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" | </table> + style="vertical-align: top; padding: 0.33em 0.66em; border-color: #A3D3FF; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" | [[MediaWiki User's Guide]] + style="vertical-align: top; padding: 0.33em 0.66em; border-color: #A3D3FF; border-style: solid; border-width: 1px 1px 1px 4px; Template:Border-radius; border-spacing: 4px; font-size: 85%" |