You may have thought that humans only have one set of lips, comprised of two separate entities, but lip piercings actually fall into eight separate categories. Lip piercing is an ancient body modification, still common among certain African tribes who stretch their lower lips to fit large, discus-shaped lip plates. The two most common piercings are done on the lip and the labret.
The basic lip piercing describes any piercing that penetrates the area directly surrounding the lips, usually along the bottom lip but sometimes found along the top as well. A lip piercing is pierced with a lip ring either directly in the center of the lip or just off to the side. It will almost always be pierced initially with a captive-bead ring, though after the wound has fully healed (which can take up to 4-6 weeks) you may later change to a stud if you so choose.
A vertical labret passes through the direct center of the bottom lip. It differs from a standard labret piercing in that a curved barbell is inserted and both ends of the jewelry can be seen--one below the bottom lip, the other in the mouth between the lips.)
A lowbret is a horizontal labret that is placed as low as impossible inside the lip, just before your chin cartilage begins.
A vertical lowbret is placed vertically inside the mouth, piercing through the cheek and emerging close to the jaw line. The barbell used for this type of piercing needs to be at least 1.5" in length.
A horizontal lip piercing, as the name implies, passing directly through either lip at a horizontal angle.
A horizontal labret piercing, on the other hand, passes horizontally just below the lip.
The Madonna piercing (yes, she gets a piercing named after her) is sometimes called a Monroe piercing, named for the original beauty mark queen, Marilyn Monroe. A Monroe is located in the upper lip, slightly off-center to mimic the look of a beauty mark or mole.
A Medusa piercing is the opposite of a labret piercing, located in the direct center of the upper lip. It is sometimes called an "upbret" or "cleft" piercing.