The Preamble of the standard describes the sheltie this way:
The Shetland Sheepdog, like the Collie, traces to the Border Collie of Scotland, which, transported to the Shetland Islands and crossed with small, intelligent, longhaired breeds, was reduced to miniature proportions. Subsequently crosses were made from time to time with Collies. This breed now bears the same relationship in size and general appearance to the Rough Collie as the Shetland Pony does to some of the larger breeds of horses. Although the resemblance between the Shetland Sheepdog and the Rough Collie is marked, there are differences which may be noted. The Shetland Sheepdog is a small, alert, rough-coated, longhaired working dog. He must be sound, agile and sturdy. The outline should be so symmetrical that no part appears out of proportion to the whole. Dogs should appear masculine; bitches feminine.