An authorized user can use the credit card but is not legally responsible for paying the bill. Parents sometimes name a child as an authorized user if they want the child to have a plastic payment method or want to help him or her build a credit history.
A joint account holder shares ownership of, and typically uses, the credit card and is legally responsible for paying the bill. Until the joint account is paid off and closed, all parties on the card are liable for the debt, even if only one party made the charges.
A cosigner does not use the credit card but promises to pay off the debt if the account holder does not. A cosigner is legally responsible for the debt, which is why anyone asked to cosign a loan or other credit account should think about the responsibility very carefully before agreeing.