When liquids and gasses flow, one of the physical laws in effect is that where velocity is highest pressure is lowest. When air flows over the curved upper surface of a flat bottom wing the air in effect flows faster over the top due to it being a greater distance, this creates a low pressure on the upper surface creating lift because the pressure under the wing is greater. This effect is only a small portion of the lift required to fly so angle of attack makes up the rest. This same effect happens in a carb because the air has to "hurry up" to maintain the same volume in the venturi as is flowing above and below the venturi. Semi symmetric and symmetric airfoils have their own characteristics.