Difference between free energy of formation and free energy of combustion
I think free energy of formation is the change in the Gibbs free energy when a compound is formed from its constituent parts (assuming that the system is in the standard state of 298K, 1atm pressure). What it means is how much work the reaction is capable of doing if it does the maximum possible work it can do (in case it's a negative number) or how much work you need to do on the system to make the formation reaction happen.

I never heard of free energy of combustion yet, but I guess it must be about the change in Gibbs free energy during a combustion reaction. So I think it's the change in Gibbs free energy for a combustion reaction when the reaction goes from the reactants to the products, and should have the same interpretation as the change in free energy for formation in general (probably the delta Gibbs value is going to be negative indicating that the reaction released a lot of energy that can be used for work).

Hope this helps...
(I'm a newb so hope others can verify my answer).