A
switch is someone who participates in
BDSM as both a
top and a
bottom. Switches are very common; partners may switch roles based on mood, desire, or being with a different partner. They might also switch because both partners in a relationship are (for instance) bottoms, and they will take turns at 'switching' to top so that they can continue their activities.
The term "switch" is also used in vanilla, especially gay sex, to refer to someone who enjoys both insertive and receptive sex. Again, this is very common.