Any aircraft that is heavier than air, can sustain flight due to its design, yet does not use an engine can be classified as a glider. There are three major categories of glider: sailplanes, paragliders and hang gliders.
Paragliders are the simplest form of glider in that they don't have a structure. They somewhat resemble a parachute and are launched from on foot, either by jogging down a hill or being towed by a vehicle.
Hang gliders operate similarly to paragliders in that the user launches on foot, but they have a rigid structure that cannot be packed down into a backpack as paragliders can. There are five different classes of hang glider, each with its own unique structure.
Sailplanes are small aircraft that have wings and a tail and are made of light materials such as fiberglass. They can be launched by a powered aircraft or a winch on the ground, but some come equipped with supplemental motors that allow them to launch without assistance.
Military branches across the world have manufactured their own gliders for dropping troops and bombs over enemy territory. These were most commonly used during World War II but were largely replaced when helicopter development reached a level of suitability for military use.