The sidebar is located on the left side of each page on the default skins. Usually the sidebar contains useful links related to the wiki and its contents. Site administrators are able to customize the contents of the sidebar by editing the system message MediaWiki:Sidebar. While the sidebar syntax might look complicated at first, it's actually very straightforward:
*header name (e.g. navigation) **page name|title displayed to users **another page|another title *another header **yet another page|and title
For example, if you wanted to have two navigation portlets, another titled "navigation" and another "for users" with links to the help page, user list, statistics and the wiki's main page, you could type something like this into MediaWiki:Sidebar:
*navigation **Main Page|Wiki's main page **Special:Statistics|Stats *for users **Help:Contents|Help page **Special:ListUsers|User list
If you want the link titles displayed to users (such as "Help page" or "User list" to be customizable, you could use system messages instead of "hard-coding" the names into MediaWiki:Sidebar. Let's have a look at another example:
*navigation **mainpage|mainpage-description *foo-header **foopage|foo-description
In this case, the messages MediaWiki:Mainpage and MediaWiki:Foopage would be used as link targets (the page where clicking a link takes you to) and the messages MediaWiki:Mainpage-description and MediaWiki:Foo-description would be shown as link titles to the user viewing the sidebar.
You can alter the location of the search box, toolbox and interlanguage links portlet in the sidebar if you're using the Monobook skin. Please be aware that most of these customizations will not have any effect in other skins.
The "magic word" SEARCH can be used to move the search box in the sidebar. If you wanted the search box to be the very first item in your wiki's sidebar, you'd type something like this into MediaWiki:Sidebar:
*SEARCH *navigation **mainpage|mainpage-description *foo-header **foopage|foo-description
The "magic word" for moving the toolbox is TOOLBOX. Most people however prefer the toolbox as one of the very last items in the sidebar, but nothing is going to stop you from moving it to be the first or second sidebar item.
If there are multiple language versions (for example, English, French and German) of the same wiki and they have same content, pages can be linked together with interlanguage links. If any interlanguage links are present on a page, these will be displayed in their very own portlet, which comes after the toolbox. You can use the "magic word" LANGUAGES to move this portlet into a different location in the sidebar.
Monaco's sidebar is slightly different as in the fact that you can't change the location of the search bar. You can change the sidebar contents using MediaWiki:Monaco-sidebar and the toolbox by changing MediaWiki:Monaco-toolbox.
The Vector skin sidebar's customization is almost the same than MonoBook with the exception of the search bar which is located on the upper right side of the page, outside the sidebar.