A terrier is a dog of any one of many breeds or landraces of terrier type, which are typically small, wiry, very active and fearless dogs. Terrier breeds vary greatly in size from just a couple of pounds to weighing over 70 pounds and are usually categorized by size or function. There are five different groups with each group having several different breeds.

Different localities raised terriers suited to their hunting or vermin control needs. Terriers were crossed with hunting dogs, fighting dogs, and other terriers. The gameness of the early hunting terriers was exploited by using them in sporting contests. Initially, terriers competed in events such as clearing a pit of rats. The dog that was fastest in killing all the rats won.Terriers range greatly in appearance from very small, light bodied, smooth coated dogs such as the English Toy Terrier (Black and Tan), which weighs as little as 2.7kg (6lbs), to the very largest rough coated Airedale Terriers, which can be up to 32kg (70lbs) or more.