A bracket link is what you get when you create a link to a wiki page by putting words in brackets.

An example:

[[BracketLink] links to the page [BracketLink].

BracketLinks allow you to deviate from the old Wiki way of CamelCase, i.e. you no longer have to MashWordsTogether to produce a link.  For example, [bracket link] is as good as [BracketLink].  This produces more readable documents.

Q:How do you place bracket in links? For instance how do you create a link to "[bug 123] links can't have brackets"? Or how do you create a link to "mailto:email@address.com?subject=[topic]"?

A: Brackets are illegal in URLs (and WikiNames), so you need to use different styles of encodings.  For example, "[" would produce a left bracket, like this: "[".  The full list is available in [http://www.w3.org/MarkUp/html3/latin1.html].

A further example.  This
{{{
[[bug 123] links can't have brackets|Bug:123].
}}}
produces this:

[[bug 123] links can't have brackets|Bug:123].