Definitely a Shetland Sheep Dog, AKA sheltie, NOT a collie. Collies have a distinctively high bridge to their nose, and they are much larger than this dog.  Shelties look very similar to collies, but are small and have a flatter face. A very large male sheltie (45 lbs) can be almost as big as a very small female collie (50 lbs) - but their faces are distinctively different.