All Russells are small terriers; Jack Russell Terriers vary between 25 and 30 cm at the withers and Parson Russell Terriers are between 32-38 cm.
They are predominantly white with black, tan, or tricolour markings, particularly on the face and the base of the tail. They have small V-shaped ears that usually fold sharply forwards, and strong teeth with a scissor bite.
They have a dense double coat that appears in three varieties: smooth coat, where the topcoat is very short (approx. 1cm) and stiff; rough coat, where the topcoat is longer (as much as 10cm long, though usually groomed shorter); and broken, which is used to describe both dogs with topcoats of intermediate length and dogs that have longer coats only on some parts of the body (always on the face, frequently on the head and back, sometimes extending to the shoulders, occasionally everywhere except the legs).
Tails are straight, six to eight inches (150-200 mm) long, held high and upright. Traditionally, tails were docked to around four inches (100 mm), the length of a hand's grip. This was supposedly to aid in pulling the animal out of a hole. In many countries, docking is now illegal, and even in countries where it is not undocked JRTs are becoming more common. The Parson himself did not dock his working terriers.
The breed has a sturdy and robust appearance and an outgoing character; breed standards emphasize that the Jack Russell must have a 'keen expression'.