The key difference is related to the licensing terms and associated support for the compilers. See the End User License Agreement (EULA) for distinctions between the three. Products under an evaluation license will cease to function at the end of the evaluation period. Free products are only valid for the granted license period. All other products will continue to function beyond the licensed support period.
An evaluation license is not renewable. Once your free license has expired, you must qualify for a new license. Paid licenses are eligible for support renewal. See the Purchase, Renew, and Upgrade FAQ for more information on renewals. Functionally, the evaluation and free tools compilers are identical to the paid compilers of the same version.