Braces are an
orthodontic appliance affixed directly to the
teeth, exerting pressure to force the teeth to shift position or alignment, primarily for aesthetic effect.
The creation of the first braces as such, metal bands encircling the teeth, are attributed to "father of dentistry" Pierre Fauchard in 1728, though human remains indicate the existence of effectively similar, if more primitive, devices several thousand years prior. Braces have developed since then to their modern incarnation, which primarily consists of metal or plastic brackets affixed to the front (or, less frequently, back) of the teeth and connected by adjustible-tension metal wires. Braces are still being refined and improved, with orthodontists and patients alike indicating that modern braces are orders of magnitude easier to install and less painful and unsightly than models from as recently as the 1970s, and their construction has recently incorporated such advances as heat-activated nickel-titanium wires, another mostly uninspiring space program spinoff technology. Unless and until straight, even teeth cease to be an ideal or an easier way to move the teeth (perhaps involving temporary decalcification) is developed, braces can be expected to remain a part of the Western dental canon.
Braces work in the same manner as foot binding, Mayan forehead-flattening, or, more acceptably to modern first-world sensibilities, the Ilizarov Procedure - reshaping of the body through constant but gentle pressure. The tension of the connecting wires is calibrated to pull the individual teeth towards the desired ultimate alignment, and braces can serve as anchors for various types of headgear and rubber bands which exert force to alter the protrusion, alignment, or shape of the jaws. Of course, given the need to perform this effect on living bone which the subject will be using on a regular basis and the resultant uninspiring magnitude of the forces involved, this process will take from several months to several years, with regular checks on progress and fine-tuning of the tension and alignment of individual components. Dentists and orthodontists typically recommend that candidates for braces have them affixed as early as possible, and as a result most wearers of braces tend to be preadolescents or teenagers, who are old enough for their secondary teeth to have come in but still young enough to be culturally and legally under the control of parents or guardians. Braces are less common but not unheard of among people of other ages, from the very young to the elderly.
In addition to the obvious aesthetically pleasing effect, the use of braces to realign the teeth may make breathing, speaking, and chewing easier, reduce the risk of injury or chipping of the teeth, and assist in the maintenance of dental hygiene. Orthodontists are known to foresee severe pain, dental disfigurement, limited mobility of the jaw, and similar outcomes in patients with "misaligned teeth" who do not undergo treatment with braces, but as always when the same person who offers the diagnosis also sells the treatment, you might want to take such prophecies with a few grains of salt. Statistically, braces reduce the risk of dental disease later in life, and there are indications that this may in turn decrease the chances of developing cardiovascular disease, but the connection is tenuous, still under study, and qualified even by the most gung-ho of orthodontists.
As for downsides, first, they hurt. When braces are first installed and every time wires are tightened, teeth typically ache for several days, and the teeth are often more sensitive throughout treatment. This was apparently much more of an issue with older braces, but having once had the new-style ones, I can testify that the pain is not yet a trivial factor. Second, braces are rather expensive - a full course of treatment, including the braces themselves, installation, tuning, and assorted orthodontic check-ups, typically runs somewhere in the mid-four-figure dollar range. Given that an array of teeth all facing directly outward will take up more space than the same teeth in their natural state (after all, crowding is one of the primary reasons teeth come in crooked), treatment with braces may require removal of some teeth to make room for the rest, and further, braces, while installed, may inhibit proper brushing and oral hygiene. As is typical for dental professionals, your orthodontist will blame any problems on your personal failures. Finally, braces are generally considered unattractive and people with braces, especially children, are often stigmatized, although in fairness, the same is just as true, if not more so, of crooked teeth in modern American society.
Personally (and myself not being a dental professional in the least, this is a philosophical, rather than orthodontic, doctrine), I consider the use of braces as appropriate, if regrettable, when used to address issues with actual medical or quality of life implications, such as when teeth on one side of the jaw do not touch, but would recommend against them as not worth the time, expense, effort, or pain when used for minor cosmetic realignment.