Retainers come in many different types, shapes and sizes. Some are just a wire attached to the teeth permanently, while others are large, colorful plastic monstrosities, it is hard to conceive that all of these appliances serve the same function in the same way. But they do, and choosing the right one may be the most important thing with regards to completing your orthodontic treatment.
The main difference between all of these different kinds of retainers is whether or not they are removable or fixed. Their appearance, although it may be very important for the patient, is secondary to the doctor, and is completely secondary with regards to the success of the treatment. The main features, in terms of functionality, is whether the retainer is fixed, or removable. In terms of function, both types of retainers do the exact same thing, in the exact same way, so the main thing is to adjust the type of retainer worn to the habits of the patient wearing them.
A fixed orthodontic retainer is the type where the device is adhered to your teeth, and cannot be taken out. This type of retainer is recommended for patients who tend to lose things, or are sloppy, have a lot on their plate and are very busy, and will have a hard time remembering where their retainers are located. Fixed appliances are just what they sound like, they are adhered to the back of the teeth, and cannot be removed. Their main downside also comes from this property, as they are fixed in a set position and may be difficult to clean, and thus may accumulate plaque and calculus, which is a bit less hygienic.
These appliances are the same as fixed ones, except they can be taken out. They can be just a wire or a complete device, they can come in many different shapes and sizes, but the main thing is that they can in fact be taken out and cleaned. Their main upside is that you can choose when to wear them and when not to wear them, and you can clean them when they are removed. Their main downside is that they tend to get lost or forgotten about. Many patients simply feel that they no longer have to put up with the discomfort of wearing orthodontics, and thus stop wearing their retainer, and once they sense their teeth moving back into place, wearing the retainer will become uncomfortable, or even impossible. This is because retainers are made from a mould that is cast after your teeth have been aligned, and are only good as long as your teeth stay in the position that they were moulded from.