To see what changes have been made to an article, you need to view the article's history.
Go to an article and clickunder Editing Tools.
This is the history of an article about Firefox Screenshots. See how the history shows the edits a person made to the article. You can also see who made the edits, the date on which the edits were made, and what the edit was for. Click on a date to see the content of the article on that date.
Sometimes when you go to edit an article, you'll see this warning message:
This tells you that there are newer, unreviewed revisions that were made to this article. Make sure that you're editing based on the latest version of the article. You can edit an article based on an unreviewed revision by clicking the pencil icon. See Edit a Knowledge Base article for details.