What exactly is a "fancy rat?"
The term "Fancy" refers to the fact that they are completely domesticated, and come in a wide variety of colours, patterns and coat types not found in wild rats.
Domesticated animals unlike wild, if released into the wild usually cannot fend for them self.
The Fancy Rat population today descended from the "Brown Rat" Latin name Rattus norvegicus, that colonized Europe in the 18th Century.
The Brown Rats are larger and bolder, thus they made better candidates for domestication. The other type of common wild rat is the "Black Rat" Latin Name Rattus Rattus. This was the rat that carried the fleas that carried the Black Plague disease all over Europe (But dogs, cats and other animals also carried fleas), these were not the Brown Rats kept as pets today, and are a different species.
The Brown Rat was first used for blood sports in the 18th and 19th centuries. Commonly called "Rat Baiting" it required hundreds of rats to be caught, then placed in an arena/ring, the spectators timed how long it took a terrier dog to kill all the rats, and made bets on such times.
Brown rats were also commonly kept and bred by Jack Black, A Royal Rat catcher to queen Victoria of England. With so many wild rats in England it was necessary to find some form of control. This led to a new career for some people, that of rat catcher. Jack Black, can be credited as the originator of the first true domestic rats. In the course of his work, when he came upon strangely colored animals, he kept and bred them. Eventually he had Albino, Black, Fawn, Grey, and Marked animals which he bred and sold as pets. Between the 1840s and 1860s Jimmy Shaw and Jack Black sold many animals and provided the background from which our present day domestic rats originate. Later on the Brown Rats (the most commonly used are the Albinos) were used in laboratories for experiments; so they have been bred down thousands of generations to be docile and friendly towards humans.
Having such a long history in the care of humans they have become docile, loving, tame and friendly animals, and are completely domesticated; Making them a very good choice of pet for older children, and quick to accept you as family. They are one of the most dog like pets (next to dogs of course!) that you can find. They do not bark, are cheaper to feed and house, and do not require regular vaccinations at a vet.