A collie is a dog whose most prominent feature is its long snout. It has remarkable intelligence and care as a herding dog -- they are the only dogs that are careful enough to herd chicks. Collies are intelligent enough that it is easy to train them with just praise, though unkind words may drive them off and abuse will drive them crazy. This dog is representative of the "look like a wolf" philosophy of herding dog breeding (as opposed to the Old English Sheepdog, which looks like a sheep).
Collies come in two types of coat length, short and rough. They have a wide variety of coloration: there is sable (the coloration most people think of when they think of a collie, largely brown on top, white on the bottom with some black mixed in on the top of the head and ears), tricolor (Largely jet black on top with a brown snout and a white underbelly), blue Merle (similar to tricolor, but with a bluish-grey mottled effect), and rarely as a bi-color or bi-black with a white belly.
Collies are very active dogs, requiring a large yard to live in and regular exercise. Collies are good with children and excellent watchdogs, though they are territorial. A collie will gently watch over a baby yet not hesitate to corner and bark down an intruder. However, they are not suited for guard dog duty, being more bark than bite. Collies shed twice a year.
The word collie/colly/colley means black in Scottish Gaelic, and that was apparently originally the dominant color of these dogs, who were first called 'Colley Dogs'.
Collies are best known in the United States through the tv show Lassie, the title character being a sable rough-coat collie who lives with a farming family.
Updates: Wuukiee tells me that
Shetland sheepdogs don't shed constantly, but twice a year, so my comment to the effect that
Shelties shed constantly has been removed.
Wuukiee also told me that they come as
bi-colors.