Plastic Surgery is broad surgical specialty that requires a minimum 5 year residency after completing medical school. The word Plastic, in plastic surgery comes from the word “plastikos” which is greek for “to mould” and it got this name because in general plastic surgery involves manipulating and moving tissue around to suit a specific purpose. Broadly speaking, plastic surgery can be divided into reconstructive and cosmetic surgery, and each of these can be further divided into numerous subspecialties that are beyond the scope of this answer.
The first descriptions of plastic surgery date back as early as the 6th century in the area that is now India. Several procedures that are still used today are described in a text known as the shushruta samhita. One excellent example is a forehead flap for nasal reconstruction, which was used at the time primarily to restore the appearance of people who had their noses cut off as punishment for various crimes. This procedure involves taking skin from the forehead and elevating it off of the underlying muscle and skull and rotating it down to reconstruct the nose. Although it is rare for someone to have their nose cut off as punishment these days, this flap is still commonly used in people who have lost part or all of their nose as a result of cancer, trauma or cocaine use.
Modern plastic surgery started as an answer to devastating injuries seen in world war one where massive traumatic defects never seen in previous conflicts required innovative solutions to repair them. The father of modern plastic surgery, Sir Harold Gilles, recognized that these wounds needed to be closed with tissue from elsewhere in order to restore a more “normal” appearance and function and allow these soldiers to live reasonably normal lives. In response to this need, he pioneered techniques that include skin grafts and flaps (the movement of tissue from one location to another while maintaining its original blood supply). A classic example of this is the large facial wounds suffered in trench warfare that were then closed with tissue from the arms or legs. These techniques were further refined by Sir Gilles and his cousin Archibald McIndoe during the second world war when once again they were inundated with soldiers suffering from devastating injuries. Injuries that if left to heal on their own would leave these individuals horribly disfigured and lacking basic functions that the rest of us take for granted (such as eyelids that close and mouths that open). This is an example of the reconstructive aspects of plastic surgery.
Once surgeons realized they could manipulate tissues to reconstruct devastating injures and restore a “normal” appearance, attention turned to manipulating tissue to take a “normal” appearing person and improve upon this in an attempt to achieve a more “perfect” appearance, giving birth to cosmetic (or aesthetic surgery). Early cosmetic surgery was very expensive and secretive and usually reserved for the ultra-rich and Hollywood stars. Popular procedures in the 1940s and 50s include rhinoplasty (nose job) and rhitidectomy (facelift) surgeries. Over time the field expanded and now pretty much anything that someone wants improved can be operated on. The most popular cosmetic surgery now is by far breast augmentation, the history of which is a long and interesting story all on its own.
In essence, Plastic surgery is like the trunk of the tree and reconstructive and cosmetic surgery are simply two branches.