InterWiki links are a quick way to refer to information in other wikis, without having to remember long or cryptic URLs. To create an InterWiki link, just use the InterWiki name of the foreign wiki, then a colon (:), followed by the name of the page on that wiki.
For example WikiPedia:MoinMoin would be rendered as MoinMoin, which points to the page about MoinMoin on the English Wikipedia site.
You can add new entries by editing InterWikiMap.
Of course, InterWiki links are not rendered differently when the remote page does not exist, and MoinMoin cannot check whether you followed the rules for WikiNames of the other wiki. So double-check what you type.
See the InterWiki page on MeatballWiki for further details.
List of valid InterWiki names this wiki knows of (the master copy of this list is at InterWikiMap):
MoinMoin marks the InterWiki links in a way that works for the ColourBlind and also is LynxFriendly by using a little icon with an ALT attribute. If you hover above the icon in a graphical browser, you'll see to which Wiki it refers. BTW, the reasoning behind the icon used is based on the idea that a WikiWikiWeb is created by a team effort of several people.