The QQQ ETF tracks the Nasdaq 100 Index. Image source: Yuya Sekiguchi via Flickr.

Millions of investors use exchange-traded funds to invest, and ETFs tracking popular indexes have gotten a lot of attention. For the PowerShares QQQ ETF (NASDAQ:QQQ), 2015 has been a strong year, with the Nasdaq-tracking ETF dominating the returns of its primary rivals. While the SPDR S&P 500 ETF (NYSEMKT: SPY) and the SPDR Dow Jones Industrial Average ETF (NYSEMKT: DIA) have struggled to remain above the break-even point for the year, the QQQ ETF is up almost 10% for 2015. Let's take a closer look at what the QQQ ETF tracks and why it has been on fire this year.

QQQ Total Return Price data by YCharts.

Parsing the Cubes
The QQQ ETF, whose shares are sometimes referred to as the Cubes to reflect the three Qs in its ticker, tracks the Nasdaq 100 index. As a result, the ETF holds the 100 Nasdaq stocks that make up that index, giving investors broad diversification that represents the makeup of the Nasdaq stock exchange.

However, the diversification that the QQQ ETF provides doesn't come close to what the SPDR S&P or SPDR Dow ETFs give. That's because the Nasdaq exchange is heavily weighted toward the technology industry, with tech companies having initially sought out listing on the computer-driven Nasdaq as an alternative to the more staid New York Stock Exchange. When you look at the QQQ ETF, you'll see that more than 55% of its value is in technology stocks. Consumer and healthcare stocks make up nearly all of the remainder. That leaves sectors like energy and utilities completely shut out of the index, while telecoms and industrials have only minimal representation.

The big winner for the QQQ
Moreover, the weightings in the Nasdaq 100 give some stocks more influence than others, and so you can generally trace the advance in the QQQ ETF to just a few stocks. The most important is Amazon.com (NASDAQ:AMZN), whose shares have doubled so far in 2015. With the stock now making up nearly 6% of the value of the QQQ ETF, that doubling corresponds to about three percentage points of the ETF's year-to-date gain.

Amazon's recent growth has centered on a couple of things. First, the company's Amazon Web Services has seen rapid growth, with revenue jumping 78% to top the $2 billion mark and making more than half a billion dollars in segment operating income. Second, Amazon continues to make progress in its core retail services division. Even though that area has never been strongly profitable, investors remain comfortable with the huge sales growth that Amazon has achieved there. Shareholders are convinced that in time, profits will come, and that will bolster the company's prospects even further.

Tech titans top the list
In addition, a couple of other big-name tech stocks have also made big contributions to the QQQ ETF's success. Google parent Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) has seen its shares jump about 40% so far this year, and combining the two share classes, Alphabet makes up about 9% of the ETF's assets. Doing the math, that means Alphabet has boosted the QQQ ETF's returns by two to three percentage points. In addition, Facebook (NASDAQ:FB) has kept climbing into the stratosphere with a 35% rise, and that has added more than a full percentage point to the ETF's total return as well.

Both Alphabet and Facebook have done well lately. Alphabet's most recent quarterly report included 13% sales growth that sent adjusted earnings up by 19%, and Alphabet has successfully pushed toward boosting its mobile-search business to go beyond its historical dominance of desktop Internet search.

Meanwhile, Facebook has reached all-time highs, going above the $300 billion market-cap mark as sales rose more than 40% in its most recent quarter. With 1.55 billion active monthly users and more than 1 billion using Facebook every day, the social-media giant has boosted its advertising revenue and has effectively captured more than its share of the mobile market.

As long as the biggest tech companies in the Nasdaq 100 keep doing well, the PowerShares QQQ ETF will be in a strong position to benefit from that favorable trend. That could give the QQQ ETF not just the title over the SPDR S&P 500 and SPDR Dow ETFs for 2015, but also bode well for future years, too.

Dan Caplinger owns shares of Alphabet (C shares). The Motley Fool owns shares of and recommends Alphabet (A and C shares), Amazon.com, and Facebook. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.