Apple Earnings: What You Need to Know

There are a few key things to takeaway when Apple reports earnings Tuesday.

(Read More: Earnings Eve: Here's What Pros Say May Turn Apple Around )

Let's start by looking at the expectations: $42.5 billion in revenue, $10.07 EPS. Not terrible, considering revenue would be three billion dollars more than a year ago. But it signals an important drop off in growth rates.

That's where we start digging in. Last quarter Apple sold nearly 48 million iPhones, up 29 percent from the year before. The average selling price of an iPhone held steady.

Contrast that with where we were a year ago: Apple was growing iPhone units 88 percent year over year.

That's what has analysts taking a sober look at the June quarter, which Apple will also forecast tomorrow: Several on Wall Street have been lowering their estimates to the mid-$30 billion range, which would be in line with Apple's seasonal patterns.

What happened?

(Read More: Why Whispers of Apple Ousting Tim Cook Are Likely Untrue)

The smartphone market has matured in developed markets like the United States and Western Europe, leaving emerging markets like Brazil, Russia, India and China as the biggest growth opportunities. In those markets, Apple doesn't have the same advantages it has had recently. Apple lists 11 stores in China, all but two in just three cities: Beijing, Hong Kong and Shanghai. Apple relies on third-party retailers in Brazil, India and Russia.

Meanwhile, Apple's next growth driver, the iPad, has run into a new breed of competition: Zero-margin hardware. Amazon and Google specifically are selling tablets roughly at cost, expecting to profit later from media, merchandise and ad sales. Apple has still managed to outsell the competition, but the low-priced rivals have likely wet customer demand for cheaper tablets and pressured Apple's margins.

So probably the biggest question mark is the iPad. Analysts expect between 15 million and 18 million units shipped in the quarter. But we'll want to see whether the unexpected downside in PC sales was matched by any unexpected upside in iPads.