Gliders utilize a combination of gravity and air currents to stay in the air. Lift, drag and thrust are also important to the gilder's ability to fly smoothly for hours after a motorized plane has towed and released them into the air.
Lift is one of the factors that works against gravity as it forces air beneath the wing. The glider is able to control lift and use it to its advantage because of the shape and surface area of the plane, as well as the angle at which the pilot is attacking the sky.
A glider's wing shape creates a situation where the air moves faster on top of the wing than it does on the bottom. Because of this, there is greater air pressure under the wing. This helps the plane lift to the sky.
When a pilot is in the air, he uses his angle of attack to get the plane up and to keep it up. The greater the angle of the wing, the more air is hitting the bottom, giving it more lift. The pilot must carefully balance the angle of attack to avoid too much lift, as this causes the plane to fly backward and crash.