Look, the link consists of three parts:
First you are telling the board HOW you want the text processed. You want a link, therefore you have to say so. Everything you put between the
will be shown as a hypertext.
Second you have to tell the board WHERE it should link, that's www.yahoo.com
Third, you have a text that should be displayed.
When you put the three together, you get something like this:
[url=www.where_the_text_leads.com]Any text you want, can even contain spaces or special letters, even a dot.[/url]