That is why, when the grill is right, that the beautiful brown stripes magically appear upon the surface of a steak. Then the exterior of the meat conducts heat to the interior, molecule by molecule. Conducting is extremely important in barbecuing because we must conduct the heat from the exterior to the center so slowly and gently that we do not dry out the exterior. Using low temperature over the long period is the essential distinction of barbecue from roasting or broiling. Incidentally, it also allows for more flavoring and more fun time.
Convection is the transfer of heat by movement of heated masses, i.e. air, water, oil.
In an oven, an enclosed grill or in the path of heated air, convection is at work. Convection allows us to remove the meat from directly over the coals and, therefore, tend the coals without disturbing the meat. It does not restrict the amount of meat which can be simultaneously cooked to the surface area of the coal bed and fat dripping from the meat does not drop into the coals. Radiation is the transmission of heat in waves of energy resulting from vibration of excited molecules. As when your tongue trembles at the taste of succulent, savory barbecue, it radiates ecstasy to your brain and other pleasure receptors.