Whenever someone makes an edit, it's put on top of a stack of edits to that page. If someone comes along and notices that the most recent edit vandalized the page, all they have to do is revert back to the previous version of the article. (See the "History" tab on any Wikipedia page for a list of edits.) That makes for a massive amount of data, which is why database dumps of Wikipedia are no less than 20 GB worth of SQL code.
Wikipedia
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