If I had a dime for every time someone asked me this question, I could stop being one. (I wouldn’t, though, because I love the work.)
A copywriter is simply a writer — a writer stuck with a silly and needlessly confusing title.
Most of the text you read in a given day — newspaper articles, billboard advertisements, corporations’ websites, sales letters, junk mail — is called “copy” in the business world. Copywriters are simply those professionals who write this stuff — mostly for businesses or other types of organizations. I write website text, press releases, articles for magazines, speeches, educational materials, newsletters — all called copy.
Now that you know what we copywriters do, you can see what a goofy job title we have. Copy means words. That means we are described essentially as “writers of words.” (As opposed to writers of something else, I suppose — lamps, for example.)
It’s an odd quirk of the profession that “copywriter” contains such an obvious redundancy right in the title — especially odd when you consider this is a profession of people supposedly good with words.
Just in case you were curious.