A traditional tongue piercing is done through the center of the tongue with the piercing located between the two muscles. The underside of the tongue is pierced, and a barbell is placed through the hole, with a ball on top and bottom to hold it in place.
A tongue webbing piercing is a piercing through the skin under the tongue, called frenulum skin. The piercing is first fitted with a ring, and can later be fitted with a barbell once the piercing has healed. Some frenulums cannot be pierced, so check with a reputable body piercer if you are interested in a webbing piercing.
A horizontal tongue piercing runs through the left to the right side of the tongue, instead of the traditional vertical placement. It is important to place the piercing properly, to ensure the tongue is still able to move correctly and the piercing does not impair speech. Most professional pierces do not perform horizontal tongue piercings.