When you start a new return, TurboTax will ask you questions about your tax situation and recommend the best product for you. As you work on your taxes, you may have to change products based on the complexity of your return. For example, if you have medical expenses, or charitable donations, then you need Deluxe or better.
Please see this TurboTax FAQ for more information on who can use Free:
Who can file for free using TurboTax?