It doesn't - at least not directly.  Here's what happens to a log after it's thrown into a fire:

1. Drying - that log is full of water.  Green wood has ~80% (of dry weight) moisture; a 9 lb log = 5 lbs wood + 4 lbs water.  Seasoned wood has ~20%, so the same log split and dried for a year will weigh 6 lbs. There's still a pound (pint) of water that must boil out before the log's temperature will rise over ~230F.  Dessication makes lots of steam that helps keep your meat moist; it also consumes a lot of heat from the fire.
2. Decomposition - once the log is dry, heat-up resumes.  The cellular structure of wood breaks down at high temperatures (400 to 800F) and boils-off much like the water did.  Instead of steam, we get smoke - a cornucopia of volatile organic compounds (VOCs) and particulates.  Smoke is a gaseous fuel.  At 800F, the log has been reduced to charcoal (carbon) as its cellular structure vaporized.  Charcoal is a solid fuel.
3. Combustion - our two fuels burn differently, each governed by The Fire Triangle.  Charcoal burns as a red-hot ember (a surface oxidation).  Smoke burns as an orange/yellow flame.  Charcoal and smoke split the fire's energy production roughly 50/50.

That simple, 3-step process has huge implications for barbecuers:

• The charcoal-only fire - charcoal is just wood that's been taken through steps 1 and 2 at a factory.  Having no volatiles, this fire is simple to control - just throttle the air supply.  This allows the pit boss to load up a surplus of fuel, dial in the temperature with dampers, and get a long burn with minimal attention.  The downside is that a charcoal-only fire is flavorless, making CO, CO2, and barbecue that tastes like pot roast.
• Smoke and creosote - the aromatics in smoke determine barbecue's flavor.  Cleanly-burned smoke makes good barbecue - that's why good pit bosses strive for clear or thin blue stack emissions.  Unburned (chalky or yellowish) smoke is the source of bad barbecue - its volatiles will condense as creosote on your cold meat, making it black, bitter, and likely to cause gastronomic disturbances if consumed in quantity.
• Power control of a wood fire - throttling air to a wood fire to control its power will produce creosote.  This is the grandest conceptual error in barbecue - pit makers design elaborate dampers and throttles, practitioners debate the merits of inlet vs. outlet throttling, novices get frustrated and give up.  The only way to make clean smoke from a typical wood fire is to maintain good, hot geometry and control the fire (and pit temperature) by rationing fuel, rather than throttling combustion air.