All goldfish are the same species, Carassius auratus auratus. Comets are a specific breed of goldfish. They have the standard goldfish body type, with a longer, flowing tail. Comets are probably among the first goldfish varieties bred. They are a "fancy" variety of goldfish, but much hardier than the more inbred varieties.
It is impossible to tell the gender of any goldfish under one year in age. However, since goldfish breed in groups, this doesn't mean that someone interested in breeding has to wait for the fish mature. Your best bet is to purchase a group of six comets and hope for the best.
In older fish, the pectoral or side fins are one of the best ways to tell the sex of a comet goldfish. The first fin ray of the pectorals in males are usually thicker than those on females. However, this does assume that the goldfish are the same age and have had the same level of care. An older, or better-fed female goldfish may have thicker fin rays than a younger male kept under sub-optimal conditions.
The best ways to tell the sex of a comet goldfish appear during the breeding season. In the males, spiky bumps appear on the gill covers and the front of the pectoral fins on the males. Sometimes, these breeding tubercles resemble the bumps and growths seen in fish diseases like ich. However, they are a good sign; a fish will only grow spawning tubercles if they are in good conditions and ready to breed.
Body shape is a more subtle way to tell males from females. When viewed from above, females are generally more wide-bodied than males. This works best around breeding season as females begin to swell with roe. This technique is less effective when looking at comets from the side; the difference is most noticeable from the top looking down.