MediaWiki is an amazing website software. It allows "wikis" to be run. A wiki is a website that anyone can edit. The reason for this seemingly strange idea is to allow the readers of the website (who use the website's content) to improve on the content as they see fit, or to fix any typos and errors only a reader, who probably probes the text patiently, and a good spell checker, will catch.
However, one of the faults of this idea is how rogue users can make malicious edits to pages. They can blank sections or entire articles, change text into offensive or nonsensical characters, or can rename pages to inappropriate titles. There's a term for this -- "vandalism".
Vandalism in real life is tedious and annoying to remove. A wall that's covered with graffiti sprayed on with spray cans will take some time to repaint. On wikis, however, reversing the effects of vandalism sometimes requires just a single mouse click...
Step 1. Open the page history
The page history stores all edits made to the article. Every time an article is edited, it's recorded, viewable, and available in the page history, along with all previous revisions made to the article. The page history is quite literally the very reason why websites like Wikipedia and Wikia can exist successfully -- if someone did destroy an article, the website will not be forever damaged.
To view the page history of a page, click on the "View History" tab at the top of the page. If you're on Wikia, the button is located in the drop-down menu next to the "Edit" button.
Step 2. Locate the revision you wish to revert to
If a user by the name User:SomeHorribleVandal attacked a page, then you'll want to look for the latest revision not made by SomeHorribleVandal. If more than one users attacked a page, look for the latest revision not made by those two users. Once you've found what you're looking for, click on the timestamp and date for the revision (looks something like "21:04, August 22, 2014").
Step 3. Edit the older revision and publish it
The Edit button at the top should allow you to edit the old revision. Click on it. You will be told that "Any changes made since this revision will be lost". That's not exactly true -- what you are doing here is essentially just bringing up an older revision, pasting it into the edit window, and then saving it as a new revision. The previous revisions will still be available in the page history as mentioned above. But, if you are reverting vandalism or another unwanted edit, this is what we want.
In your edit summary, you should probably state that your edit is a revert. You can either just write "Revert" or the abbreviation "RV". If you are reverting vandalism, you can write just that, or use the abbreviation "RV/V". If you are reverting an edit that is not vandalism, it's best to write a descriptive explanation as to why you're reverting the edit.
Once you are done, click on "Save page" or "Publish". That's it!