Retainers are used to keep teeth straight or to make minor changes.  There are several types and many variations.  Removable retainers can be, uh, removed, and are generally either clear (like aligners) or with a visible wire (classic type, see pictures).  Fixed retainers are attached to the teeth and cannot be removed unless you really, really want to take them out, i.e. with a pair of pliers (not recommended).  Which type you need depends on several factors, such as how your teeth looked to begin with, what types of movements or changes have occurred throughout your treatment, your age, your personality (not kidding, somewhat), and your doctor's personal preference.  I will let you know at the beginning of your treatment which type I expect you will need.