"This door closed" means it closed of its own accord: I let go of it and Bang! it closed.
"This door is closed" describes the state of the door: I may not have seen it in the act of closing, but it is now closed, and if I want to go through it, I have to open it first.
So the first sentence describes an action in the past, and the second sentence describes a state in the present.
"This book sells well" means that lots of people buy this book. "This book is being sold well" means that the PR agency are doing their job encouraging people to buy the book.
- For Teachers