A breed of dog. Rat terriers are generally small but very muscular, with coloration mostly in black, white, and tan. They usually come in three sizes: the standard is about 14-23 inches tall and 12-35 pounds; the mid-sized terrier is 8-14 inches and 6-8 pounds; and the toy is about eight inches tall and 4-6 pounds. Their tails are often docked at birth.
The rat terrier was probably developed in Great Britain in the 1820s and brought to the United States in the late 19th century. It gets its name from its skill at catching and killing rats and other vermin, which made it a popular farm dog in the early years of the 20th century. It was also an excellent hunter of small rodents like rabbits and squirrels. It was probably developed from crosses between English white terriers, Manchester terriers, smooth fox terriers, and whippets, with later genetic additions from beagles, Italian greyhounds, miniature pinschers, and chihuahuas. They are considered very similar to small mixed-breed hunting dogs called feists. The breed has benefited from a lot less of the inbreeding that most other breeds go through, so they're generally healthier and smarter than many purebred dogs, but they are a bit neurotic and excitable.
Rat terriers make great pets -- they're smart, active, and very loving. They're pretty territorial, so they make good watchdogs, but once they're properly introduced to strangers, they will usually accept them. They are extremely energetic, so they need a lot of exercise -- you should try to give them at least a half-hour's worth of exercise a day. They like to dig, though, so you'll have to take precautions to make sure they don't dig under your fence. They love to play and are considered fairly easy to train. Their life expectancy is 15-18 years.
Nipper, RCA's trademark dog listening to the old-timey phonograph, was probably a rat terrier, and Theodore Roosevelt almost certainly owned some and helped popularize the breed name. The breed is not currently recognized by the American Kennel Club as a purebred dog -- their genetic diversity works against them in that respect -- but the breed's popularity makes it likely that they will eventually be recognized by the AKC.
My brother's dog, Hyla, is a rat terrier, and I love her dearly. I am her indulgent uncle who lets her misbehave, so she takes my visits as her excuses to bark loudly and jump in the air until I sit on the couch so she can crawl in my lap and step on my crotch. When she finally calms down, we love to sit together and contemplate how awesome we both are. She loves a nicely carpeted house because carpets give her the traction she needs to race around at ungodly speeds. Hyla has a colossal appetite, and before my brother learned to keep food off the kitchen countertops (she has little trouble jumping up there if there's something there she really wants), she managed to eat everything from a box of prunes to most of a loaf of French bread to an entire apple, core and all. Luckily, her hyperspeed metabolism keeps her trim. She is impossibly cute, and I love her so.