A Lip piercing is a type of body piercing which penetrate the lips, or the area surrounding the lips. Lip piercings can be placed anywhere around the mouth, but the surface of the lip is not typically pierced itself, except for horizontal lip piercings and canine bites. Piercings in specific positions have certain names:

Monroe piercings are labret studs worn on the upper lip where Marilyn Monroe had her famous mole. This piercing is also known as the Madonna piercing in some circles.
Medusa piercings go through the center of the upper lip (the philtrum), perpendicular to the tissue.
Labrets are pierced with a labret stud. A flat disc sits between the back of the lip and the gums and a bead sits on the outside of the lip. Labrets can pierced in the center or off-center. A variation of this is the lowbret, a lower labret.
Vertical labrets go through the center of the bottom lip, parallel to the tissue. The variation is called the vertical lowbret, which starts inside the mouth between the lower lip and the teeth (not behind the teeth as with a mandible piercing) and travels straight down, exiting on the lower edge of the jawline.
Horizontal lip piercings are a horizontal bars on the lower lip that go through the lip surface. This is a very rare piercing.

"Bites" is a term for sets of lip piercings placed in certain ways. There are many types:

"Angelbites" or "Antibites" are two upper lip piercings on opposite sides of the mouth. They are sometimes made up of two Monroe piercings
"Snakebites" are two lower lip piercings on opposite sides of the mouth.
"Venombites" or "Viperbites" are two lower lip piercings that are closely placed side by side.
"Caninebites" are four piercings directly through the lip, one on each side of the mouth. In other words, the act of getting both "Angelbites" and "Snakebites".
"Spiderbites" are three piercing across the lower lip one left off center, one in the center, and the last right off center.
"Cyberbites" are when the middle labret piercing and the medusa piercing are combined.

Lip piercing continues to be practiced by several peoples, the most well-known of which are certain African tribes, who wear large decorative lip plates or discs, usually in the lower lip.

In contemporary society, lip piercings are relatively common piercings, especially amongst youths.

Here you can find some PICTURES OF LIP PIERCING.