Although Apple (NASDAQ: AAPL) sells a wide range of products, analysts, investors, and the media seem to focus heavily on just one of the company's product lines: the iPhone. And there's good reason for that. I'd like to go over why the iPhone business is so critical to Apple and discuss some of the basics that current and potential investors in Apple stock need to know about it.During Apple's fiscal year 2017, the company raked in about $229.2 billion in revenue.