That of an active, game, hardy, small working terrier of the short-legged class; very free in its movements, strongly but not heavily built, standing well forward on its forelegs, deep in the ribs, well coupled with strong hindquarters and presenting a well-proportioned build with a medium length of back, having a hard, weather-resistant coat; head shorter and wider than any other terrier and well furnished with hair giving a general foxy expression.

CLARIFICATION:
This first sentence describes the essential picture of "a big dog in a small package".

The Cairn is one of the short-legged terriers, but a working terrier, not so low to the ground as to have a "weasely" look yet not too high on leg as to appear "up on stilts".

The word "medium" is the key word throughout this Standard. Not a short back, square-looking terrier, but a terrier with enough length of back to be able to twist and turn in and out of holes and among rocks with quick movement is ideal.