In the page history for any page, click (prev) (#9 on the image top right—click on the image to see a larger version) to see the changes for any version from the one just before it. To compare a series of versions, just click any two radio-buttons to select the set of versions, then click the Compare selected versions button (this only works if you have JavaScript enabled).
You should be presented with a screen that shows you which words have been removed and which ones have been added (affected words are highlighted in red). You always can revert to an earlier version by selecting it from the history, editing and saving it. We call these "diffs" because the URL has the word "diff" in it. Diff is short for difference.
