A description by Mrs Ruth Hussey-Wilford, Secretary of The Parson Russell Terrier Club.
The Parson Jack Russell

Small white fox-working terriers were bred by the Reverend John Russell, a parson and hunting enthusiast born in 1795. In his last year of university at Oxford he bought a small white and tan terrier bitch called Trump from the milk man. Trump was purchased based upon appearance alone. (Burns, 2005) She was the basis for a breeding program to develop a terrier with high stamina for the hunt as well as the courage and formation to chase out foxes that had gone to ground, but without the aggressiveness that would result in physical harm to the fox, which would have ended the chase, and so was considered unsporting. The line of terriers developed by John Russell was well respected for these qualities and his dogs were often taken on by hunt enthusiasts. It is unlikely, however, that any dogs alive today are descended from Trump, as Russell was forced to sell all of his dogs on more than one occasion because of financial difficulty, and had only four aged (and non-breeding) terriers left when he died in 1883.
The only painting that exists of Trump was painted more than 40 years after the dog died, and it was painted by someone that had never seen the original animal at all. Russell said the painting was "a good likeness" but in fact he may have been trying to be polite, as the painting was commissioned by Edward VII (then Prince of Wales) who befriended Russell in his old age, and had the painting done as a homage to the old man. (Burns, 2005)
On the day that the impoverished Rev. John Russell died, his old sermons and other papers were found blowing around in the farm yard. Little or no written record of Rev. John Russell survives to the present day.
While it is often stated that Trump was "14 inches tall and weighed 14 pounds," there is no source for this statement, and it appears to have been penned by someone who had never met Russell and had only seen the painting of Trump (to which there is nothing to suggest scale). (Burns, 2005)
While Trump's appearance is murky, and her size a complete mystery, the fox dens of Devon, England, where John Russell once hunted, are well known. Terrierman Eddie Chapman, who has hunted those same Devon earths for more than 30 years, notes that "I can state categorically that if given the choice, ninety-nine percent of hunt terrier men would buy an under 12" worker, if it was available, over a 14" one." (Chapman, 1994). To this day most working terrier enthusiast seem to prefer a dog around 12 inches tall and with a chest span of around 14".
Jack Russells that are not trained and exercised regularly may exhibit unmanageable behaviour, including excessive barking, escaping from the yard, or digging in unwanted places inside and outside the house. In America, several Jack Russell rescue networks have to work constantly to find temporary and permanent homes for Jack Russell Terriers whose owners typically were not aware that Jack Russells are not "docile" dogs and could not meet these requirements. Prospective Jack Russell Terrier owners are advised to be responsible.
Most Jack Russell Terriers get along well with children so long as they are introduced carefully, but they are extremely protective of their territory and have no tolerance of even unintentional abuse. Most are outgoing and friendly towards other dogs (again, territorial invasions notwithstanding), but a good number show same-sex aggression issues, especially the males. JRTs are also known for a "Napoleon complex" regarding larger canines that can get them into dangerous situations. Their fearlessness often scares off a larger animal, but their apparent unawareness of their small size can lead to a lopsided fight with larger dogs if not kept in check.
It is not uncommon for a Jack Russell terrier to be cat-aggressive (although they have been known to get along with them over time in the same house) and homes with other small fur-bearing animals in them (pet hamsters, rabbits, etc) would do well to think through the ramifications of bringing a JRT into the house as their hunting instincts are strong
The Reverend Jack Russell did not have Jack Russell terriers – he had white-bodied fox-working dogs that, in his day, were simply called “fox terriers.”
The term “Jack Russell Terrier” was coined after the Reverend John Russell was dead, and was used to differentiate small working terriers from over-large non-working Fox Terriers that by 1900 dominated the Kennel Club show ring and bench.
Today, the term "Jack Russell Terrier" is used to describe a wide array of dogs. Though there is a difference of opinion as to what is a “true” Jack Russell Terrier, it is revealing that the Reverend John Russell himself, never registered his own dogs with the Kennel Club and described his own dogs as being very different from those found on the show ring bench: "True terriers [my dogs] were, but differing from the present show dogs as the wild eglantine differs from a garden rose."
The simplest way to think about Jack Russell Terriers is to divide the entire lot of them into two groups as John Russell himself did: Those that actually work in the field, underground, to formidable quarry (what Russell himself valued), and all the rest -- pets and show dogs alike.
Such a simple demarcation stood for more than 100 years, but ended in 1990 when The Kennel Club (UK) decided to add the Parson Russell Terrier to its rolls. The American Kennel Club followed suit in 2001, as did the United Kennel Club that same year.