There are two main ways to launch a glider. The first method is by winch. This involves a wire being pulled in fast, taking you up like a kite. The second method is by aerotow, where a small aeroplane called a tug tows you up into the air. This is best demonstrated with pictures:

Winch Launch (Pulled up like a kite, on a strong cable). Glider goes up at about 45 degrees. (Photo could be better...)	Aerotow Launch (Towed up by tug aircraft (in front of glider's left wing in photo).
K8 Single seater being winch launched.	Glider pilot's view of the tug in the air. (The yaw string on the glider canopy got in the way!)

A similar, alternative method to winch launching is tow-car launching. A Tow vehicle races along the runway, pulling the glider behind it, which is connected by a long cable. Obviously it is less powerful than a winch launch, so not as much height is obtained. This method used to be used years ago at Lasham (long before I was there) before the winches were bought.

I have also heard of another method of launching called 'Bungee Launching'. I have never been launched via this method, but it involves attaching the glider to the middle of an elastic rope, streched from two anchor points (one for each end of the rope). This method only tends to be used on a windy day, and the glider is launched into ridge lift, which is caused by the strong wind being deflected over a ridge or hill. I guess it's probably the closest you can get to being launched by rubber band!