You may have heard about “Invisible Braces”.  They are not really invisible but they are certainly less visible than metal fixed braces.  They are made of clear plastic and fit tightly over your teeth.  They are designed to apply pressure to the teeth in-order to move them.  You are usually provided with several of these aligners as a new one is fitted every 2 weeks or so.

They are more expensive than fixed braces and are only available if you pay privately for your treatment.