First we should know more about this dog breed, Rat Terrier is an American dog breed, one of many terrier varieties most of which are closely related to the feist. It is a small, active terrier.
Rat Terrier comes in a variety of coat colors and patterns. It is often white with either black and tan, apricot, blue, or brown spots. The most common color is the tricolor: black, tan, and white. Many have ticking on the white part of the coat; most have freckles on their bellies. Ear carriage can be upright, tipped, or button, and the tail is traditionally docked to about 1/3 of its original length.
Rat Terrier ranges from about 2 to 40 pounds, although the larger sizes are sometimes called Deckers or Decker Giants, after a breeder named Milton Decker who has been developing a larger version of the Rat Terrier.

And if we talk about temperament, Although often mistaken for a Jack Russell Terrier, the Rat Terrier has a very different temperament. It is less aggressive and comparatively calm (although still exhibiting terrier personality). They are normally cheerful dogs, but there are occasional dogs who are extremely sensitive to changes in their environment or to unexpected noises, people, or activity. They can be, however, extremely bonded to and protective of their owners.

When searching for a Rat Terrier be sure to find a breeder who does temperment testing to determine if the individual dog will suit your home and family. Rescuing a Rat Terrier is a good alternative to contributing to the sometimes cruel breeding process. If looking for a rescue Rat Terrier be sure to ask lots of questions about the dog you are looking at. Answer all questions that the rescue organization asks you so that they can determine the right fit for you.

source; wikipedia