As most parents will probably tell you, that ink is not "supposed" to be in your body. Turns out, your body itself would agree. When a needle breaks your skin, your immune system interprets that as a wound and sends defensive cells to the site in order to heal it. The same occurs when a needle that's loaded with dye penetrates past your epidermis (your top layer of skin) and deposits that dye into your dermis. Your immune system sends cells to the "wound" — some of which try to eat away at the dye, while others absorb it. It's these latter cells, called fibroblasts, that make your tattoo visible and permanent. After soaking up the dye, these cells stay suspended in the dermis, deep enough that they aren't affected when you shed dead skin.
Click through to the next page for the TED video to see what this process looks like and how it can be reversed. You know, in case you come to regret that "Spirng Break 2007" tat.