Few technological revolutions have gripped the world in the way that smartphones have. You could rightly argue that the PC revolution decades ago was similar in many ways, but at the same time it was fundamentally different. There are still many developing countries where a smartphone or other mobile device will be a consumer's first computing device.

A total of 1.3 billion smartphones were shipped in 2014, a healthy 28% higher than in 2013. Even as some developed markets have become saturated and sales are transitioning to upgrade cycles, there are still plenty of first-time adopters out there.

Predictably, the smartphone market faces many of the challenges that the PC market did, albeit at a faster pace. Manufacturers must combat the hardware commoditization that inevitably erodes pricing power, which puts the vast majority of hardware companies at a distinct disadvantage. It should come as no surprise, then, that the three main stocks to watch in the smartphone market are Apple (NASDAQ:AAPL), Google (NASDAQ:GOOG)(NASDAQ:GOOGL), and Samsung (OTC:SSNLF).

Why you should watch Apple

Apple's iPhone 6 and iPhone 5 lines. Source: Apple.

Since Apple embraces an integrated approach with software and hardware, it can differentiate itself from the competition and command unrivaled pricing power. It's no coincidence that the iPhone has single-handedly made Apple the most valuable company in the world. Even though Apple might not be the No. 1 smartphone vendor by unit volume (that title still goes to Samsung), it is easily the most profitable.

As global smartphone prices trend lower, Apple has remained resilient. Apple enjoys the most loyal customers, who also happen to be more affluent and willing to upgrade to the latest and greatest devices. The company is heavily reliant on its iPhone business, which now comprises nearly two-thirds of its revenue. The smartphone market is evolving, but Apple is keeping up handily.

Why you should watch Google
In no uncertain terms, Android has become the Windows of the mobile era. Google's smartphone platform has become nearly ubiquitous around the world, and the search giant has countless hardware partners willing to churn out an endless number of devices that appeal to any imaginable consumer or market segment. Android's truly disruptive nature comes from being an open-source OS and the fact that original equipment manufacturers, or OEMs, don't have to pay any explicit licensing fee.

That's not to say that implementing Android is "free," though, since manufacturers must devote plenty of developmental resources into bringing a device to market. Google also has plenty of hurdles for OEMs to clear before they can get the search giant's blessing in the form of Android certification.

The shift to mobile has weighed on Google's business in a number of ways, however. Most notably, ad prices are lower on mobile compared to desktop platforms by virtue of the reduced display sizes. Still, Google can make up for that with volume, and it has navigated the transition well. Even though Google does not explicitly disclose Android-related revenue, investors can rest assured the OS remains critical to the company's future.

Why you should watch Samsung
Samsung's handset division has had a couple bad years as Apple has gained share in the high end while the South Korean conglomerate has been marginalized in the low end. Yet Samsung has one key advantage that many of its rivals (including Apple) lack: incredible scale and vertical integration. While Samsung still relies on Google for Android, the company makes virtually all of its own hardware components. On top of that, Samsung has extensive relationships with carriers all over the world, ensuring a vast distribution network that outstrips all others.

One significant challenge for U.S. investors is that Samsung is not easy to invest in. Not only does the Lee family still collectively exert an inordinate level of control over the company, but Samsung's shares aren't listed on any major domestic exchange. Instead, shares trade over the counter. Trading over the counter entails its own risks in the form of low liquidity and wide spreads, so investing in Samsung directly isn't for the faint of heart.

However, interested investors can take a position in Samsung through vehicles including mutual funds or ETFs that have large positions in the company. For example, Samsung has a nearly 21% weighting in the Deutsche X-trackers MSCI South Korea Hedged Equity ETF.