Lassie was what is known as a regular Collie of the rough variety. This means that she was a Collie who had a full coat with long, luscious hair. But there are other kinds of Collies out there. If you hear of Smooth Collies, it means that they're also of this breed but have a short coat. Same breed. Different variations!
Thanks to the Lassie movies, novels, and tv shows, rough-coated Collies became the most famous variations of the breed. People were after dogs with the Lassie signature coat. With time, they've become more and more common to the point where if you think of a Collie, you're probably thinking of this variety and not of the smooth-coated Collie.