The end

What do you see on the screen at the end of a film/movie?

So it it's "the end", why then do we talk about a happy ending, a tragic ending, a Hollywood ending?

Basically, we say "the end" when we are talking about the final point in time or space. "The ending" refers to the story and what happens in it. This is why we use adjectives like happy, sad, strange, confusing, etc. with "ending".