Angle of attack
The angle of attack is the angle between the chord line of an airfoil and the direction in which the aircraft is moving (relative wind). Along with changing the airspeed and transforming the shape of the wing by using flaps, changing the angle of attack will either increase or decrease the amount of lift that is generated. An increase in the angle will result in greater lift. However, if the angle is increased too high, it could result in a loss of lift and lead to a stall. The unwanted effect of an increase in the angle of attack is the increase of induced drag, although it does not increase as much as lift. This is true until about 12 degrees, when drag starts to increase dramatically. As the angle increases, a separation of airflow from the upper surface of the airfoil becomes prominent and lift actually begins to decrease. The angle at which the airfoil is no longer able to support the weight of the aircraft is the critical angle of attack.
The angle of attack is not to be confused with the angle of incidence. This is the angle that is measured between the longitudinal axis, running through the length of the aircraft, and the chord line of the wing. On most aircrafts, the wing is inclined relative to its fuselage, so there is a slight positive angle of incidence.