A diff is the difference between two versions of a page. It can be viewed from the page history: for every version there are potentially two radio buttons: the left column is for selecting the older version, the right column for selecting the newer one. Clicking "Compare selected versions" gives the difference between the two versions.
For special cases (the diff for a single edit or between an old and the current version) other possibilities are 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.
- Help:Diff on Wikipedia, upon which this page was based.