The colon trick is a method of providing a link to a category, image or interwiki link without adding the page to the category, displaying the image or adding the interwiki link to the interlanguage links. It also allows for linking to pages with titles beginning with a recognized external link prefix, such as http:// or //Hus.
By adding a colon just inside the opening double square brackets of the link, the default behaviour of the MediaWiki software will be bypassed. It will appear as a conventional wikilink.
[[Category:Help]]would list this page in Category:Help, but
[[:Category:Help]]will produce a link to Category:Help.
[[File:Wikipedia-logo.png]]would display an image, but
[[:File:Wikipedia-logo.png]]will produce a link to File:Wikipedia-logo.png.
[[pt:Internet]]will create an interlanguage link, but
[[:pt:Internet]]will produce an interwiki link to pt:Internet (in the Portuguese Wikipedia).
[[//Hus]]will produce an (invalid) external link wrapped in brackets ("[]"), but
[[://Hus]]will produce a link to //Hus.
[[Computer]]links to the Computer article; adding a colon like
[[:Computer]]can be done, but the colon is unneeded and makes no difference.
A colon can also be used to transclude a page in mainspace, rather than template space. For example,
((The Beatles)) will transclude Template:The Beatles, but ((:The Beatles)) will transclude the mainspace article The Beatles. This is useful when the mainspace article has been specially formatted to be transcluded in other articles, by using <noinclude> or <includeonly> tags to indicate the portion that should be included when the article is transcluded.