Tell me, what's the difference between a writer and a copy editor?

Tweet

I've been puzzled by this kind of work since both are needed in making an article. For me, I guess a writer is the one who would select a topic of a particular article and writes a very good story content. While a copy editor, will do the editing like checking the errors of the writer which includes the grammar, spelling, punctuation, but not changing the thought the the writer wants to present. So, how about you can you elaborate more on what's the difference between a writer and a copy editor?