|The 3-tier diff and link series|
A diff is a permanent link to a single edit or set of edits, showing the difference between the before and after versions. Without diffs, those trying to help you may not know what you are referring to. 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 told "Diffs or it didn't happen."
This is how to make a diff:
To properly link a diff, paste it into your text where you want it to appear. Include all parts of it, starting from
https://en.wikipedia.org/w/index.php?title=. Put single square brackets, [ ], around it.
[https://en.wikipedia.org/w/index.php?title=Main_Page&diff=212488423&oldid=210742813] will show up as  and will link to the chosen diff of Wikipedia's Main Page.