What exactly is the "door shell" on a car?

The door shell is the outside skin only. You would not have much luck if you tried to replace it yourself. The skin is crimped over the edges and spot welded. The repair shops have the tools, equipment and expertise to do this.

Car Door Shell

Well, something kind of similar did happen to me last month. I was in my house getting ready to go somewhere when a gypsy man knocked on my door and told me he could fix the huge dent in the side of my car for only $100. My racist side said, "He's a gypsy, don't do it! He'll rip you off!" (actually, this wasn't so much racist as factual. There is a group of gypsies in this town who survive by scamming people. A couple people in my neighborhood have fallen prey) Then I thought to myself, "But I love the gypsies! They have such an interesting culture!" (they do). And so I said, "Sure!!" And he went to his car and pulled out an old milk carton full of chemicals and a hammer, and set to work. An hour later, my car had been poorly spray painted and bashed about a bit, and I was down $100, but actually I was pleased because you can hardly tell that there is a dent at all.

the shell is the outside of the door.