The domestication of rats began to occur over a century ago, rat catchers who were employed to keep the rat population at bay used to catch rats of unusual color, many of these would be kept for display at public houses. Today's pet rats are usually descendants of the brown rat rather than the black variety, and now a variety of colorings are available. Since the early 1970's keeping rats as domestic pets has again become popular, the facts about rats are now widely known and as such unwarranted fear of disease spreading has all but gone.
The facts show rats to be extremely intelligent rodents and as such will respond well to handling and will over time become quite tame, it's important you get your pet rat when they are very young, this will make them easier to tame. Rats rarely bite unless they are either frightened or badly handled, so it's important to ensure you take suitable care.
Size wise, rats will grow fairly quickly and well need increasingly larger accommodation, they are likely to grow to around 10-12inches in length and have a life span of around 5 years or so. When deciding to keep a rat as a pet it's important to be aware that their urine can be rather strong, although this is not so extreme with females. It is also important to keep rats as singles, do not keep more than one in the same enclosure, although this isn't a totally solid rule, so well tamed rats can live in pairs or even trios.
Rats are nocturnal rodents but will also be active for some parts of the day. In terms of different varieties available, there are not more than around 30, and many of these are no widely available. If you do seek some of the more unusual color rats you may need to consider contacting a breeder or at least a specialist in this field. The most popular type of rat tends to be the white or albino rats.