From Online Manual
description: The url tag makes http hyperlinks. If a single url is placed inside the tags, then a link will be made to that url. These links will normally open in a new browser window, however this behaviour can be overuled by a user's browser settings. For links that will open in the same window by default, use the iurl tag.
Links with descriptions can be made using this format. This is an inline tag and no block level tags can be placed inside, however other inline tags can be if nested correctly, with the exception of the email, ftp, iurl and url tags.
URLs added to post text, with no URL tag, will be auto-converted to hyperlinks.
example: [url=http://simplemachines.org]text or more BB Code[/url]
result: text or more BB Code