Activator is an appliance used to posture the lower jaw forward.
Headgear is an orthodontic appliance that helps in guiding the mandible growth, and is typically used on growing patients to correct skeletal problems. This is done by slowing the growth of the upper jaw, and hence allowing the lower jaw to catch up.
The headgear can also be used to manage space for teeth in case of a crowded maxillary arch.It is then attached to the molars (via molar bands & tubes), and helps to push them or draw them backwards in the mouth, opening up space for the front teeth to be moved back using braces and bands.
Headgear needs to be worn approximately 12 to 22 hours a day to be truly effective in correcting the problem, and treatment is usually anywhere from 6 to 18 months in duration, depending on the severity of the problem and the extent of cooperation of the patient.