|See? It's all the same thing.|
The base coat is the coat of clear polish that goes on your nail before any color or other polish goes on. You do this to protect your nail.
The top coat is the coat of clear polish that you put on after all other polish has been painted on.
Get it now? They're both the same substance, made of the same things. It's just when you put it on that changes the name of it.
Hope this helped you save some money! It's a good thing to know.