The Jack Russell terrier has a double coat—predominantly white with black, tan, or black-and-tan markings—that is harsh and weatherproof and may be either rough and wiry, broken (intermediate), or smooth. About the size of a fox, the Jack Russell terrier stands 12 to 14 inches tall (30 to 35 cm) and weighs 13 to 17 pounds (6 to 8 kg). Its legs are longer than those of many other terriers, enabling the dog to pursue its prey on foot. It has a “button ear,” which folds forward; its tail is docked to a few inches, traditionally left long enough to provide a handhold to pull the dog from a fox's burrow. The breed is noted for its tenacity, courage, energy, and strong hunting instinct. In 2003 the breed's name was changed in the American Kennel Club standard to the Parson Russell terrier, the name by which it is known in the United Kingdom.

The “shortie” or “pudden” Jack Russell terrier has shorter legs, standing 10 to 12 inches (25 to 30 cm), is smaller overall, weighing 11 to 13 pounds (5 to 6 kg), and has a somewhat different ancestry. In the United Kingdom the breed is designated the Russell terrier.