What is the difference between having a co-signer and becoming an authorized user?

To be a co-signer, both people need to be over 21; to be an authorized user, you can be any age

As a co-signer you are responsible for the bills; as an authorized user you are not

With a co-signer, you can open a credit card; as an authorized user, you can open any type of credit account

As a co-signer, you open an account and an adult signs on with you; as an authorized user, you are added to someone’s existing account