Interwiki links are links to other (wiki) sites not hosted by ShoutWiki, such as Wikipedia and other Wikimedia Foundation websites. Interwiki links allow you to create a link to such a wiki without having to use the normal external link syntax.
[http://en.wikipedia.org/wiki/Foo page title] vs.
Interwiki links should not be confused with interlanguage links, which basically are links to the "same" wiki in different languages.
Interwiki links are shared between all ShoutWiki sites, so for example the
[[wikipedia:Page name|Link title]] link syntax will work on every ShoutWiki site. Interlanguage links, on the other hand, are specific to one wiki.
You can view both all available interwiki prefixes and interlanguage links on Special:Interwiki. Only ShoutWiki staff may add, edit or remove interwiki prefixes and interlanguage links via that special page.
Please contact ShoutWiki staff with your request.
Right now there is no strict criteria for interwiki links, but it definitely helps to state your reasons why you'd want that particular wiki to be added to the interwiki table. In most cases, you can use normal external links or a template to link to the wiki.