Referral is a registered user who earns for you. There are 2 (two) kinds of referrals:
Direct Referral:A direct referral is someone who uses your username in referrer field while registering. 


Rented Referral:
A Rented Referral is a Referral you paid for, Mostly for one Month with a Later Extention if you Like. Prices can be Different and vary from Site to Site,