What does an undelivered letter look like and what scenarios are there?

I've looked on Google Image Search, but I can't find anything definitive. Is there a postal stamp on it? Or a sticker? Can someone send me a picture/link of it?

And how can a letter be "undelivered"? I read online that the post office tries several times and then it's returned to the sender. But suppose the post office puts it in a mailbox that's broken and the wind blows it out into the street. And someone finds the letter and takes it to the post office or drops it in a post office mailbox? Does it then also get returned to the sender as undelivered, or does that only happen if the postman takes it right back? Are there other scenarios for undelivered letters?