The main difference is that an original is made by the company that launched that watch, with the standards of quality, sourcing of materials, and ethical work conditions with fair wages assured by the fact that it comes from a well-known and respected brand. And this is one of the main reasons why watches are so expensive. Or at the very least "original" watches. With a replica, none of that happens.
Also, while an original can enjoy the support of the house and receive replacement parts, obviously a replica cannot - and in some countries, possession of such a watch could be considered a crime. Obviously, laws vary by country, so if you're not sure, the best thing is to check how things work in your country.