The key distinction is that a lighter than air craft relies on aerostatics (aka boyancy) to stay in the air, while a heavier than air craft must rely on air movement.
This means that in still air, a lighter than air craft can in-principle stay airborne indefinitely without motive power.
In practice most lighter than air craft are unpowered balloons. It is possible to create a lighter than air craft with engines to provide propulsion and this is known as an airship. Airships have however remained relatively rare, they tend to be relatively slow, have small payloads for their size and be sensitive to wind.
A heavier than air craft on the other hand requires power to maintain altitude in still air. An unpowered heavier than air craft in still air will lose altitude. A typical glider tow apparently drops the glider off at about 1000 feet and a typical glider apparently sinks at about half a meter per second. That gives a flight time in still air of about 10 minutes.
Gliders can remain in the air for longer through exploiting rising air currents from thermals or ridge lift. However these sources of natural lift are unreliable and location dependent.