A diff is a permanent link to a single edit or set of edits, showing the difference between the before and after versions.

If you contribute an argument to a page like requests for comment, requests for arbitration, or the incidents noticeboard for administrative attention, it's essential to give evidence for your claims in the form of diffs and/or other links. Without such evidence, you will be taken less seriously. If you simply claim that "editor X has posted several personal attacks on talkpages" or "editor Y is edit warring on George W. Bush", you may be ignored, or told "diffs or it didn't happen".

The most useful evidence consists of diffs to individual edits and links to page sections. Don't link to entire talkpages, as they are too long and diverse to be helpful. Avoid linking to a page history or an editor's contributions, as those will probably have changed by the time somebody clicks on your link to view them.

Find the page which contains the edit you want to refer to. Click on the View history tab. Find the edit in the history list. (If that's a problem, clicking on the word prev in the list will let you read the edit.) Right-click on its "prev" button and select "Copy link address" or "Copy Shortcut". The diff you want is now in your clipboard.

Find the page which contains the section you want to refer to. Click on "Permanent link" in the "Toolbox" in the lefthand sidebar. Go to the page's Table of Contents. Right-click on the name of the section you want to use, where it appears in the Table of Contents, and select "Copy link address". The section link you want is now in your clipboard.

Diffs and other links are all formatted in the same way. The whole of them must always be used, including the https://.

1. Diffs and links will work if they're simply pasted "naked" into your text like this: https://en.wikipedia.org/w/index.php?title=Wikipedia:Simple_diff_and_link_guide&diff=797180441&oldid=797180310.
2. Or you can make them neater by putting single (not double) square brackets round them. Typing this: [https://en.wikipedia.org/w/index.php?title=Wikipedia:Simple_diff_and_link_guide&diff=797180441&oldid=797180310] will give a note like this:[1].
3. Or you can make them elegant by wrapping them into a word in your text. Typing this: [https://en.wikipedia.org/w/index.php?title=Wikipedia:Simple_diff_and_link_guide&diff=797180441&oldid=797180310 this] will give this link. Notice the space between the link and the wrapper word.

Other ways to represent diffs are using the {{diff}} template (i.e. {{diff|prev|797180441}} resulting in [2]) or using special diff links (like [[Special:Diff/797180441|this]] resulting in this). Please see Help:Diff for more details.