The difference between a CD account and an online savings account is the ability to access your money and the interest rates between them.
An online savings account is an account where you deposit or put money into for savings and with the online savings account you cannot easily withdraw your money and also online savings accounts pay less interest.
So with an online savings account you earn less money on the interest while with a CD account you earn higher interest on the money you deposit and keep in their.
Also with a CD account your money is more liquid meaning that with the CD account you can withdraw your money more often without much if any penalties and still earn higher interest.
However if you're saving money in a savings account you must keep the money in the savings account or face penalties for early withdraw.
So you could lose money by withdrawing the money too early but not with a CD.
A CD account is much better than an online savings account or any savings account because it earns you higher interest and if you happen to need some money you can withdraw it.