For jet engines, it is often confusing to remember that aircraft thrust is a reaction to the hot gas rushing out of the nozzle. The hot gas goes out the back, but the thrust pushes towards the front. A glider is a special kind of aircraft that has no engine. Some external source of power has to be applied to initiate the motion. During flight, the weight is opposed by both lift and drag, as shown on Vector Balance of Forces for a Glider.
Paper airplanes are the most obvious example, but there are many kinds of gliders.
Some gliders are piloted and are towed aloft by a powered aircraft, then cut free to glide for long distances before landing. The motion of the airplane through the air depends on the relative strength and direction of the forces shown above. If the forces are balanced, the aircraft cruises at constant velocity. If the forces are unbalanced, the aircraft accelerates in the direction of the largest force.