An invisible Essix retainer is a clear plastic aligner covering all tooth surfaces to maintain their position. They usually last 6-12 months or more, and are easily removed.
A Fixed lingual retainer is considered the gold standard of orthodontic retention. A thin wire is attached directly to the inside surface of teeth. It can last indefinitely, does not hinder speech, and is not visible to others.
The Hawley retainer is removable, adjustable and durable. It can last for many years. It is plastic or acrylic, with metal wiring. In the long run, this is the more affordable option.