When is a Retainer Used Rather Than Braces?

Hi there,
Retainers keep teeth in the position they were put in with braces.  The word "retain" means to keep something in position.  Retainers come in various shapes and sizes, from those that are clear and plastic, to the ones that seat into your palate with a bar across the front teeth, to ones made entirely out of metal (Cobalt-chrome retainers).  Retainers are ineffective in moving the teeth and are used for a period determined by the orthodontist AFTER the teeth have been straightened using braces.
As for braces, these are active appliances which straighten the teeth by application of forces.  Braces can either be fixed or removable and can be either visible or almost invisible.  There are types of braces, called clear aligners, such as invisalign, which look like retainers that are worn after braces.  Invisalign moves the teeth by changing aligners every week so that each new aligner has a slightly different tooth position, thus moving the teeth gradually with each change.  Other types of removable brace includes simple removable appliances with an acrylic plate.  Fixed appliances are fixed onto the teeth, they usually have brackets that are mounted onto the teeth on the inside (lingually) or outside (buccally) of the teeth and effect their action by the use of wires which are changed according to the prescription of the treating dentist. 
Hope this helps!
Best,
MJ x

Retainers are designed to retain the final position of your teeth at the end of your orthodontic treatment. In order to move your teeth into new position/alignment you would need a series of "retainers" where the tooth position is altered in between each aligners. An example of such treatment would be Invisalign, or ClearCorrect. Talk to an orthodontist and he or she can tell you how many trays you would need. Good luck!