There are two methods of launching gliders we use frequently at our local airfield.
Gliders are often launched using a stationary ground-based winch mounted on a heavy vehicle. This method is widely used at many European clubs, often in addition to an aerotow service. The engine is usually a large diesel, though hydraulic fluid engines and electrical motors are also used. The winch pulls in a 1,000 to 1,600-metre (3,000 to 5,500 ft) cable, made of high-tensile steel wire or a synthetic fiber, attached to the glider. The cable is released at a height of about 400 to 700 metres (1,300 to 2,200 ft) after a short, steep ride.
Winch launches are cheaper than aerotows and have the advantage that many members of a club can be taught to operate the equipment. A winch may also be used at sites where an aerotow could not operate, because of the shape of the field or because of noise restrictions. The height gained from a winch is usually less than from an aerotow so pilots need to find a source of lift soon after releasing from the cable, otherwise the flight will be short. A break in the cable of the weak link during a winch launch is a possibility for which pilots are trained. A video showing this launch method can be found here.
In an aerotow a powered plane is attached to the glider with a tow rope. Single-engined light aircraft or motor gliders are used. The tow-plane takes the glider to the height and location requested by the pilot where the glider pilot releases the tow-rope. A weak link is often fitted to the rope to ensure that any sudden loads do not damage the airframe of the tow-plane or the glider. Under extreme loads the weak link will fail before any part of the glider or plane fails. There is a remote chance that the weak link might break at low altitude, and so pilots plan for this eventuality before launching.
During the aerotow, the glider pilot keeps the glider behind the tow-plane in either the "low tow" position, just below the wake from the tow-plane, or the "high tow" position just above the wake. In Australia the convention is to fly in low tow, whereas in the United States and Europe the high tow prevails. One rare aerotow variation is attaching two gliders to one tow-plane, using a short rope for the high-towed glider and the long rope for the low tow. The current record is nine gliders in the same aerotow.
There are other ways of getting a glider into the air, less commonly used nowadays.
Used more in the early days when gliders were not launched from airfields but instead homebuilt wooden aircraft were taken to the tops of hills and launched by muscle power alone. The principle is similar to a winch launch but with the steel cable replaced by bungee rope, and the V8 engine replaced with a team of strong, sympathetic helpers.
There is one place in the UK where you can have a go at this type of launch. The Long Mynd in Shropshire, is a frequented expedition site and offers bungee launching when it gets windy enough. A very gentle and graceful way of getting airborne, true to the spirit of gliding. This video shows a bungee launch at the Long Mynd.