What is the main difference between cosmetic and plastic surgery?

Beauty vs transform. Whereas cosmetic tends to refer to beauty treatments the term plastic encompasses both aesthetic and reconstructive treatments.
Overlapping fields. Plastic surgery is a greek term meaning, "to mold or change". Within the breath of this huge field is cosmetic surgery, which enhances, alters and rejuvenates the body. All good plastic surgery is a combination of alteration and improving upon what is touched.
Reconstruction. Over the years, primarily due to the media, "plastic surgery" has become synonymous with "cosmetic surgery". However, in reality plastic surgery actually includes both cosmetic surgery (to change or improve physical appearance), and reconstructive surgery (eg for birth defects, after trauma or cancer).
Cosmetic surgery. Plastic surgery refers to surgical techniques that reshape and rebuild normal structures. This includes reconstructive surgery to rebuild damage or missing parts of the body. Cosmetic surgery involves using surgical techniques to enhance the appearance of parts of the body.
Good question. Plastic surgery includes a number of areas: cosmetic surgery, hand surgery, craniofacial surgery, burn surgery, microsurgery, reconstructive surgery (e.g. Breast reconstruction after mastectomy, etc.) and, more recently, composite tissue transplantation (e.g. Hand transplantation.) cosmetic (or aesthetic) surgery involves starting with a normal feature and trying to improve it.