Q-switched and Pico lasers, are the only lasers that can remove tattoos. These lasers penetrate deep into the skin with minimum risk of forming scars and changes in pigmentations.
Q-switched laser produces an intensive focused light beam that affects the pigment in the tattoo. Afterwards, the pigment absorbs the light energy, which causes it to break into a large number of tiny particles. The body removes those particles by the process of phagocytosis.
You can find more information about this procedure on Laser tattoo removal page.