Buoyed by a collection of high-flying internet and technology stocks, the NASDAQ Composite has recently been hitting record highs. Of course, that is good news for a slew of exchange-traded funds (ETFs). That includes the Invesco QQQ ETF (QQQ), the ETF tracking the NASDAQ-100 Index. The NASDAQ-100 holds the 100 largest non-financial stocks listed on the NASDAQ. This is one of the indexes where legendary names such as Apple Inc. (AAPL), Microsoft Corporation (MSFT) and Amazon.com, Inc. (AMZN) reside. With a roster like that, it is not surprising that QQQ is one of the largest equity ETFs and a popular alternative to other broad market funds, such as S&P 500-tracking ETFs.

QQQ, which actually holds 107 stocks, celebrated its 18th birthday in March. Today, the ETF is one of the most heavily traded ETFs in the U.S. and has $49.2 billion in assets under management. Up 16 percent year to date, QQQ is trading in record territory as well, but some analysts are expressing concerns about how much further the NASDAQ-100 can rally. Additionally, some analysts are concerned with QQQ's concentrated lineup. (See also: What Is the QQQ ETF?)

Technology is the largest sector in the S&P 500, but QQQ devotes 57.7 percent of its weight to that sector. That is more than double the S&P 500's technology exposure. Including Apple and Microsoft, seven of QQQ's top ten holdings are tech stocks. "While the fund's technology orientation won't always pay off, it has worked out well over the past decade, owing to its overweighting in the technology sector and more favorable stock exposure in that sector," said Morningstar. "However, its sector concentrations tend to make it more volatile than most of its large growth peers." (See also: PowerShares QQQ Trust ETF.)

In addition, QQQ is a cap-weighted ETF, meaning that the biggest stocks by market value command significant chunks of the fund's roster. For example, just two stocks – Apple and Microsoft – combine for 20 percent of QQQ's weight. QQQ also devotes just over 22 percent of its roster to consumer discretionary stocks, but nearly one-third of that exposure is dedicated to one stock – Amazon. "This weighting approach increases the fund's exposure to stocks as they become larger and more expensive, and reduces its exposure to stocks as they become smaller and cheaper, which may have higher expected returns," said Morningstar.

QQQ's third largest sector weight is healthcare, but the bulk of the ETF's healthcare holdings are biotechnology stocks. Biotechnology is usually one of the most expensive parts of the broader healthcare sector. Over the past decade, QQQ has outpaced the Russell 3000 Index by 520 basis points, according to PowerShares data. (See also: Smartphone Markets Are Maturing: Should You Still Own QQQ?)