A diff (from the word "difference") is a web page displaying the difference between one version and another of a Wikipedia page. A particular version of a page is called a "revision"; each revision has a unique date and time of creation which can be seen in the page history. Clicking a "diff" link in a page's history will display the diff of two successive revisions.
Using the history of a page, it is possible to generate a diff between any two versions – select the left radio button for the older version, the right radio button for the newer version, and click Compare selected revisions.
While editing a page, the Show changes button displays the changes you are about to make. The diff is also shown during an edit conflict.