System messages (alternatively called "MediaWiki messages") provide a means for customising the interface of a wiki. Each system message has a predefined value that can be overridden by editing the appropriate page in the MediaWiki namespace.
When MediaWiki was first coded, the need for the interface to be heavily customizable quickly became apparent. Many wikis wanted to tweak certain words to incorporate wiki-specific terms, for example using the term "Scribble" versus "Edit" on creative writing wikis. Foreign language wikis needed a way to quickly translate terms from English into their native language.
The solution was system messages. Practically every word you see as part of the interface - from the "Edit" and "History" buttons to the navigation elements to the global footer - is customizable.
When an extension or feature is introduced to MediaWiki, Wikia staff give each piece of its interface a pre-defined English value that can be overridden by editing the local message in the MediaWiki namespace. A list of all system messages on your wiki is available at Special:AllMessages.
While the messages are there for the purpose of customization, please keep in mind that it is not always wise to tweak these messages. The default value is there because it is considered the most universally understandable terminology. So while "edit" may make more sense to say "revise", "scribble", "tweak" or something to one wiki, "edit" in general is considered the most straightforward wording that invokes to the average user what the edit button will actually do when clicked.
Because the messages are stored in the MediaWiki namespace, only admins and bureaucrats of a wiki have the necessary permissions to be able to make changes to the system messages.
Go to Special:AllMessages and select the "all" radio button. Search the default text and locate the message you would like to change. Click the message in question.
If the message has never been edited before, you will see the editor prefill with the default message value. This will help confirm you are editing the correct message.
Edit the page just like a normal page, putting in the message text. Tip: Pay very careful attention to anything with a "$" in the message. This designates a system variable that will fill in with extra text when the message is actually being invoked. It is best to seek outside help from the Community Central Forums if you are unclear what these variables represent.
Save the message and verify the message has properly changed. Tip: - Clear your browser cache (Ctrl + F5) if you do not immediately see the change.
Using the special language code trick
Prior to MediaWiki 1.18, the only way to find the name of the system message page was to browse Special:AllMessages manually. MediaWiki 1.18 introduced a special 'language' code "qqx", which can be enabled by appending ?uselang=qqx to the URL (see this page as an example). All the messages on that page will then be replaced by their message keys, which are the name of the page in the MediaWiki namespace.
Notable System Messages
MediaWiki:Wikia.css - Controls the CSS look of the Wikia skin.
MediaWiki:Mainpage - Controls the landing page when a user goes directly to the wiki in their address bar.
MediaWiki:Licenses - Defines the options users see for image copyright status when uploading an image.
MediaWiki:Edit - By default, "edit", this message controls the text one clicks when they wish to edit a page.
MediaWiki:Newmessageslink - Customizes what the new messages bubble says.
MediaWiki:Newarticletext - This is the message that you get when you follow a link to a page that doesn't exist yet.
Mediawiki:Viewsource - A message of the view source message when a user is blocked or a page is protected.