Referral is when you send your friend an email to tell him/her to come sign up for TF. If they sign up and write your name down as the person who he/she hear TF from, you get a referral.
And sometimes we have had referral contests to see how many people a member could refer in a certain amount of time. Of course, the referred person had to make a few, non-spam posts, and there were a couple of other rules, but it was interesting.
Maybe we can do that again sometime in the future.