Let's help them along a bit then with some real info instead of this watered down errm... stuff:

http://www.faqs.org/rfcs/rfc1738.html

Which has since been superseded by

http://www.faqs.org/rfcs/rfc2396.html

and in turn by:

http://www.faqs.org/rfcs/rfc3986.html

I think that's where we are right now, I might be wrong.

Happy reading.

Incidentally, the article refers to URLs, instead of URIs:

http://en.wikipedia.org/wiki/Uniform_Resource_Identifier

URLs are a subset of URIs, the differences are subtle for the most part you can ignore the existence of URNs and pretend URL == URI but it is useful to understand how they relate and that there is a difference.