Gliders fly in the same way as aeroplanes, the shape of the wing produces a low pressure about the wing as the glider moves through the air in. This low pressure is known as lift. Gliders don't need an engine or wind to be able to fly as they are either launched using a winch or towed into the air by an aircraft.
Gliders can stay in the air for prolonged periods by flying through areas of rising air. This is either "dynamic" or "thermic" lift, produced by topographical features or by heating (usually from the sun hitting the ground).
For more information there is an excellent summary here: http://www.wolds-gliding.org/about_gliding/fly_main.htm