Let's use a current example:
I have adored reading since I first learned how. I have also had multiple favorite books over the years. Harry Potter, The Vampire Diaries, The Gemma Doyle Trilogy, and Pretty Little Liars fill my shelves. My friend recommended Pretty Little Liars to me during summer camp and I quickly became absorbed in the plot. By the end of the session I had read the first three books and upon returning home, I bought and read the fourth. The fourth book was unbelievable I loved it from the beginning to the dramatic conclusion. I was overjoyed when the very end of the book made it clear that there would be more novels. The next four books were gradually released with tantalizing waiting periods in between. Every now and then I would get to read one of the books a day early due to my consistent pestering of the staff at the book store. The end of the 8th book was perfect and deep down I knew that it was the end of the series. Imagine my joy when I later discover that my now finished series is to become a T.V show!
I was severely disappointed. The T.V show started with promise but quickly unraveled in a series of some small and some not so small changes. Huge plot twists meant for the third book were revealed at the end of the first few episodes. The way the writers were burning through plot I knew the series wouldn't possess integrity long. A few new characters and dragged out plots later I make a new discovery. There will be 4 more books in the series. The purpose was as clear as day; these books were meant to milk the cash cow. To hook T.V fans on the book series. And of course to prolong the plot of the T.V series by four books. At this rate I fear that Pretty Little Liars will become more known for the T.V antics than the epic series. Much like how Vampire Diaries has warped itself into creating brand new characters going by Elena, Bonnie, Stefan, and Damon. This is what I'm talking about. The T.V show is still called Pretty Little Liars, Vampire Diaries, etc... but it has become a different thing completely.
I suppose this is where the saying don't judge a book by it's movie derives from.
If this concept is still a little fuzzy let me elaborate:
Let's say that you and your friends have an adventure. You go to the beach and uncover a buried chest filled with sand. While you are annoyed that the chest was empty, the joy of finding the chest was enough. You walk home and spend all night playing chess. The next day you discover your adventure will be made into a movie! You get all excited and a year later you sit down in the theater to see what you've been waiting for. Wait a minute, that's not right. My friend loved math, not soccer. And hey! There wasn't a treasure map in that chest on the beach! And we were never captured by pirates! Whoa, I didn't kiss that guy in real life. And why are we all telling stories by the campfire? We played chess, not truth or dare! What is this?! Right before your eyes you see that in an effort to make your story more interesting it has been completely altered. And yet it will always be known as your story despite the major differences.
This happens everywhere. This happens every time a movie or T.V show is based on a novel or a true story. What you once cared so much about has been turned into a horrible mess of action, new characters and romance. And the worst part of all is that sometimes you are tempted to enjoy it.