In affiliate programs, it is the terms of service between the advertiser and the affiliate, which defines and governs the affiliate relationship. The affiliate agreement will specify what both parties are responsible for, payout terms and commission for conversions. Most affiliate programs will ask you to agree to the affiliate agreement before submitting your application to join the program.
See "How Affiliate Marketing Works" in the "Did You Know..." section of Webopedia.
Also see Refer-It.com, a directory of referral & affiliate programs for e-commerce, featuring information on over 6,500 affiliate programs.