What exactly is writing well? Is it writing with perfect grammar? Not necessarily.
Is it writing that's worthy of The New York Times bestseller list? Maybe, but I'm not sure how practical that is.
Could it be as simple as writing so that your readers understand the meaning you're trying to convey? Yep, I'm pretty sure that's it.
So how do you make sure your readers understand you?
First, you have to determine the purpose of your writing -- the message that you're trying to get across to your readers. That may sound simple, but it's not always so. What are you hoping to accomplish and how will you do it? The four main purposes in writing are to entertain, to inform, to persuade, and to question. Which purpose are you writing for? How would an article for a teen magazine on the dangers of drinking and driving differ from an article in a professional medical journal on the effects of alcohol on a person's cognitive ability? What about a blog post detailing your weekend escapades at a local bar with your friends? Think about how the content, tone, vocabulary, and style would vary in each of these formats. Each piece might deal with the effects of drinking, but from a very different perspective and with different, even competing, interests in mind.
Next you need to think about your audience. What do they already know about your subject? What do they need to know? Why does this particular audience need to read what you're writing? A reader already familiar with the problems of overdevelopment in her small town probably doesn't need a long backstory on what caused the explosive growth. She wants to know what's going to happen next, how other towns have responded to similar problems, and what she can do to reverse the damage.
Now you know what you're writing, why, and for whom. That will get you a long way, but you're not done yet; you have to be effective. Your audience may be crystal clear on what you're trying to say, but the trick is to get them to care. You may have great information on today's financial state and how boomers can successfully plan for retirement, but if you can't capture -- and keep -- their attention, that information never gets read.
You want to appeal to your audience and deliver your message in a way that rouses them to action, or makes them think, or laugh, or cry; you want to make an impact. That's where style and voice come in. Style covers things like word choice, the types of sentences you write, how you introduce a piece, how you conclude it, and how you show your meaning (if you don't know it already, memorize and make your own the mantra of "show; don't tell"). Voice is that element that's intangible, yet so crucial for effect. Voice in writing is what makes it yours and no one else's. It's you peeking out from behind your words saying, "This is me!" It's what gives your writing power and captivates your readers.