The particles of pigment that make up tattoo ink are too large for the body to breakdown, hence its permanency. Lasers work by producing an ultra fast pulse of light energy which shatters these large ink particles into smaller pieces that the body can remove through its natural elimination process.
After treatment, the body begins to attack the smaller particles of pigment and the tattoo begins to fade over the next few weeks. Once the body has removed all ink it is able to, another treatment is then possible.
This procedure is repeated until the tattoo has faded sufficiently for a replacement piece, or until the tattoo has completely gone.
The amount of treatment required varies significantly from person to person depending on their own metabolism and the type/colour of ink, however the first few treatments provide the most reduction and the darker ink is the easiest to remove.