A "barbell" is any piece of jewelry consisting of a central metal rod, called a shaft, capped on both ends by a metal "bead" or "ball". One or both beads may be screwed on or off, but it is common that one end is welded or glued solid either before or after its sale as it is not usually necessary that both beads be removed by the user. As simple as this design may seem, there are many variations on the form, some functional and others more aesthetic. Beads may take any number of shapes, including cones or flat caps, though most are spherical. Many shaft variations exist to accommodate particular locations of the body, such as the eyebrow or a flat region of skin, as in a surface piercing. When you choose your jewelry, be sure to consider the tension it may place on whatever part you hope for it to inhabit. Excessive strain on the skin can result in what is known as "migration", during jewelry is gradually pulled to the surface of the skin. This can eventually result in the loss of your piercing and often leaves unsightly scars on or around the site of your piercing.

Barbells are distinguished as either "externally" or "internally" threaded, the former being the most common. The shaft of an externally-threaded barbell is threaded with a spiral at one or both ends so that the bead or beads can be screwed on through a hole drilled through their center. This is perhaps the simplest variety, mechanically, but irritation caused by the threads of the shaft (which must of course go through the piercing) can cause unnecessary inflammation and impede the healing of the wound. A rarer sort of externally threaded barbell is known as a "step-down barbell", in which the threaded portion is thinner than the rest, resulting in less irritation. These are less common, being produced mostly by high-end manufacturers.

Internally threaded barbells have a flat shaft which is hollow at the end or ends. The beads, instead, have an appendage that is threaded to fit the middle of the shaft so that the threading does not come in direct contact with the piercing. This variety has become more common, but is usually more expensive, its cost reflecting the additional complexity of the design. That said they are in almost all ways preferable to their counterparts.

Other variations are based on the shaft, which can be curved or twisted for aesthetic purposes or (more commonly) to alleviate skin tension. Twisted or spiral barbells may be worn in the ear lobes, but are usually inappropriate for other parts of the body. A barbell meant for the eyebrow or navel may have a curve of forty-five to ninety degrees, a feature intended to reduce the tendency of a straight bar to pull at the skin when it moves while displaying both beads most visibly. Surface piercings, which are made through a relatively flat section of skin such as an area of the back or neck, are frequently filled with "surface bars." These are shaped something like your standard staple, with a ninety-degree bend before each end of the shaft, and are meant to alleviate the especially strong skin tension produced by surface piercings, which have a high rate of migration. When choosing an item of jewelry for a body piercing, be sure to consult your piercer or another trustworthy source as to precisely which style of shaft is suitable for you. The most stylish variation may sometimes be the worst choice for your piercing.