New book - 'Building a Better World in Your Backyard' - on Kickstarter (sponsored friend)
Diffs are pages that show the differences between page revisions in a wiki page.
A diff page shows 2 columns with the text removed on the left and text added on the right, and the more recent revision of the page displayed below the columns.
To view a diff, go to the history tab, select two revisions and click "Compare selected revisions".
The diff page is sometimes hard to read - especially when looking for a very small change, or when the software has trouble aligning the equivalent sections. If you need to find the difference, try copying the relevant "before" and "after" paragraphs into Diff Checker.