Everyone who can write can write, right? Yes and no. (And for those who want to break down the word "everyone," let's not get caught in that web.)
You see, copywriting breaks a lot of the rules we learned in school. There are several types of writing: academic writing, copywriting, content writing, business writing, technical writing and legal writing, just to name a few. Each one has its own style and implications.
This is the type of writing that we're taught in school. It tends to be grammatically formal. No contractions, spell out numbers through nine, etc. Most of this is guided by a style book. The style book is guided by your discipline. Broadly speaking, English majors know MLA, while science majors look to APA.
In business writing, the purpose is to communicate professionally and clearly. Think emails, memos, proposals, etc. (Grant and proposal writing also have their own subsets of rules.) How formal your business writing is varies by industry and by relationships.
Finally! I've seen a few different copywriting definitions, but my favorite comes from Copyblogger, who defines it as the art and science of strategically delivering words (whether written or spoken) that get people to take some form of action. In copywriting, we're free from the constraints of the grammar rules pounded into our brains as children, because our strategy is to be conversational. Because conversations vary by audience, knowing to whom we're writing is crucial.
One of the biggest challenges I face is convincing people (who probably did well in school), that we don't have to adhere to academic standards when copywriting. Let me give you an example: I once had a heck of a time convincing a manager that not only was it ok to use contractions but it was also, given our audience, essential. My audience consisted of parts manages and service managers in automotive dealerships. These people are extremely busy. They're numbers people. They need their information fast and concise. We needed to speak to them on their level, like humans. When we write formally to a group of informal people, we risk condescension. Not a very motivating strategy.
And, yes, I just used a fragment, because copywriting incorporates creative writing, and if you know anything about creative writing, it's that anything goes.