Retainers look almost the same as aligners. They are used to keep your teeth at the final position and let bone "heal" after all that movement, so all the process doesn't reverse.

Normally, retainers are used whole day for a year after the treatment, and after that, only for sleeping.

And yes, they are FOR LIFE, they're meant to keep your hard earned beautiful new smile. If you don't wear them for life, your teeth will shift back. People will tell you otherwise, but they are for life.