The Jack Russell Terrier is a small terrier that has its origins in fox hunting in England. It is principally white-bodied and smooth, rough or broken-coated and can be any colour.

Small tan and white terriers that technically belong to other breeds are sometimes known erroneously as "Jack Russells". Each breed has different physical characteristics according to the standards of their national breed clubs; size and proportions are often used to tell them apart. Some authorities recognize a similar but separate breed as the Russell Terrier – a shorter-legged, stockier dog, with a range of 20–30 cm (8–12 in). However, the Fédération Cynologique Internationale (FCI) regards the Russell terrier as a sub-type of Jack Russell terrier. Jack Russells are also frequently confused with the Parson Russell Terrier. Technically, the Parson Russell is usually larger and officially limited to a middle range, with a standard size of 30–36 cm (12–14 in), whereas the Jack Russell is a broader type, with a size range of 25–38 cm (10–15 in).

Jack Russells are an energetic breed that rely on a high level of exercise and stimulation. They are relatively free from any serious health complaints. Originating from dogs bred and used by the Rev. John Russell in the early 19th century, from whom the breed takes its name, the Jack Russell has similar origins to the modern Fox Terrier. It has gone through several changes over the years, corresponding to different use and breed standards set by kennel clubs. Recognition by kennel clubs for the Jack Russell breed has been opposed by the breed's parent societies – which resulted in the breeding and recognition of the Parson Russell terrier. Jack Russells have appeared many times in film, television, and print – with several historical dogs of note.