A switch is a person that can enjoy both the submissive and dominant role within a short period of time. Which role the switch assumes can be determined by a lot of different factors. Some switches change purely by their mood, others are dominant toward one sex while submissive to the other. A person that changes between being dominant and submissive only rarely and then stays in that role is not usually considered a switch. The process of turning from a submissive to a dominant is often called topping out while the reverse is called bottoming out.
Switches are often seen as people that can’t make up their mind or too strong to submit and too weak to dominant. This is similar to the view of bi-sexuals in the vanilla world.