During the retention phase you'll be fitted with a retainer. There are two types of retainers:
Removable retainers: these look like thin sports mouthguards which fit over your teeth. They need to be worn 24 hours a day for the first three months, then at night thereafter. You can remove them for eating and cleaning your teeth but, to ensure your teeth stay in their newly straightened position, it's vital you wear them as instructed.
Fixed retainers: these consist of a very thin metal wire which is attached to the back surface of your teeth. It is permanently fastened so can only be removed by your orthodontist but it won't interfere with your lifestyle or your diet. By sitting on the tongue side of your teeth, noone can see it but it does the job of keeping your teeth straight.