a referral link is one where you usually get credit on a website for bringing traffic to it. Usually there's a userid or number in the link which identifies it as a referral link. for example,
http://www.somesite.com/?id=34234. I'm sure you know what standard links are.
At least, that's my understanding of the rule.
Here's another example of a referral link...
http://www.programmingtalk.com/showthread.php?t=24455 I'm sure an admin will kick his *** shortly.