An authorized user is another individual that you allow to charge on your account. 

Authorized users maybe your spouse, your child or others.  Businesses also establish credit cards for their employees, who are authorized users.

Depending on how the account was opened, authorized users maybe jointly and severably responsible for the account.  That means if one individual doesn't pay the bill, then the other(s) are responsible.  In this situation, the tradelines are reported on all the individuals who are authorized to use the account.  They essentially become co-signers on the account.

If push comes to shove and you are taken to court, there have been cases where authorized users were found not be liable for the debt because they didn't receive/purchase the goods. However, once again, it all depends on what the agreement said.