In the old days we used winches or bungees to launch gliders and sailplanes off the sides of a hill. Today we still use winches but aerotows are more common. A small powered aircraft tows the sailplane upwards using a 200' long polypropylene rope, about 3/8" thick. Once the planes reach 2000' (the common height for release) the sailplane releases the rope and begins what is generally referred to as "free flight".

Glider on Tow