It's important to have a good mix of accounts on your credit report, including primary tradelines AND authorized user tradelines.
Credit is defined as "The ability of a customer to obtain goods or services before payment, based on the trust that payment will be made in the future."
Credit cards are a great way to build a positive credit history. However, they can get you into debt if you are not responsible. Keep your credit balances low, under 10% no higher than 20% as you build your credit.