"Lassie is a fictional female Rough Collie dog character created by Eric Knight in a short story that was later expanded to a full-length novel called Lassie Come-Home."

There are many types of Collies, but the Rough Collie (also called "Long-haired collie") is one of the most well known. It's a medium to large sized breed that was bred for the purpose of herding in Scotland, largely popularized by Lassie.

More Info: en.wikipedia.org