When you get a tattoo ink pigments are inserted into the skin. The immune system senses this, and attempts to remove the ink, but not with much luck. The ink pigments are huge, so difficult to budge, and this is essentially why tattoos are permanent.
To remove a tattoo, this process is accelerated - really accelerated. Lasers (powerful beams of light) break apart the large ink pigments, allowing the body to process them. How this works is really just an energy transfer: the ink absorbs energy from the laser.
Current laser tattoo removal involves really high levels of energy. When the ink absorbs this energy, it heats up rapidly and expands, damaging the surrounding skin - often causing blisters and burning. Eventually, the expansion will cause the pigments to break apart, but not without causing long term damage first.
