What's the difference? I think these two sentences seems slightly different. But I don't know the exact difference. Could anybody explain the difference to me?
The door was open.
In this sentence 'open' is an adjective. It means 'not closed or blocked up'. The sentence has the same structure as 'the boy was smart' or 'the girl is beautiful'.
The door was opened.
This sentence is a passive. It means 'somebody opened the door' in an active voice. Here 'open' is used as a verb. 'Open' is used in its past participle form. I will give you a few more examples:
The door was opened. -> Somebody opened the door.
The food was eaten. -> Somebody ate the food.
Both sentences have the same structure.
The first sounds incomplete. Ideally, it would be followed by a reference to the person who opened the door.
The door was opened by Peter.
This is the passive voice of the following sentence: "Peter opened the door." (this is active voice).
The second is a standalone sentence. It just means the door was open, and not closed.
It looked like I'd rung the bell unnecessarily. The door was open.
Should someone ask you "How did you get in here?" and you tell them "The door was open," they'll nod sagely and let you get on with whatever you were doing before they came in.
However, should you reply, "The door was opened," they'll compliment you on your fantastic English, after which they'll ask you, out of curiosity, what country you're from.
That, in a nutshell, is the difference. However, the other answer is also valid.