Apple's credit plan is initially for a "secured" purchase (the bank retains the right to repossess your computer until it is paid off). These credit arrangements are easier to get than unsecured credit. You guys may qualify for secured credit where you would not qualify for unsecured credit. Consider putting it in your name, with your mom as guarantor (she promises to pay if you do not). You can do this in most states when you are 17, and it starts you on the way to having your own credit history.
If you can pay each and every payment in full and on time or early, there is no finance charge. If you miss a payment or do not pay off the full amount on time, they charge very high interest, ALL the way back to day 1.
Be sure to buy Applecare for that portable computer to cover most repairs (except a broken screen) while you are paying it off. Put the Applecare on the same bill so you can pay it in payments as well.