The basis for flight is creating forces perpendicular to the motion of the aircraft. This is accomplished in gliders with the wings. As we drag a wing through the air, it is mounted on the body of the aircraft (fuselage) such that air is deflected above and below the wing. The angle that the wing is mounted to the fuselage is known as the angle of incidence. A scientist name Bernoulli found that air following such a path accelerated over the top of the surface and that the faster moving air had a lower pressure relative to the slower moving air. In the case of airfoils or wings the slower moving air is found underneath the wing. The effect was to cause a lifting force upwards perpendicular to a line moving through the centre of the wing (the chord).
For wings both lift and drag are produced. Drag are related to the shape of the aircraft, skin friction and the like is known as parasitic drag and drag related to the angle with which the aircraft and wing moves through the air and produces lift is known as induced drag. The more the wing is angled into the the wind (angle of attack) the more lift is produced and consequently more induced drag.
The efficiency of a wing is improved if it is shaped such that the air wants to flow in attached layers over its surfaces. When this is achieved the flow is said to be laminar and drag is minimized. If the wing is angled such that the airflow becomes turbulent, then much less lift is generated and the wing is said to be stalled. The angle at which the wing becomes stalled is termed the critical angle or stall angle and in straight and level flight is represented by a certain airspeed know as the stalling speed Vs. An aircraft is unstalled by decreasing the angle of attack below this critical angle (usually around 18 degrees).