According to my knowledge, heat is nothing but the result of the vibrations of atoms and molecules. I guess this mean that in heating up a gas or liquid, we are increasing the rate at which the container of this gas or liquid vibrates, thereby imparting to the molecules a certain amount of energy.
How would this work in case of a burning log in the fireplace? Is it vibrating and thus heating up the molecules in the room? Or is it releasing highly energetic photons that do the required bombarding? What exactly is happening when we light a fire?