More new investors are getting into the stock market than ever before, and a lot of the credit for that should go to Robinhood. The app-driven stock brokerage platform has attracted many first-time investors, and Robinhood has made stocks and stock-holding exchange-traded funds accessible to many who'd never considered them in the past.

On Robinhood's list of 100 most popular stocks, you'll find five tickers that are actually ETFs. Below, we'll give the basics on these five funds, with the goal of helping you decide whether they're worth considering as part of your own investment portfolio.

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1. Vanguard S&P 500 ETF

Vanguard S&P 500 ETF (NYSEMKT:VOO) has a very simple objective: to match the performance of the S&P 500 Index. It rises and falls in line with the broader stock market, as the S&P 500 owns about 500 of the largest stocks in the U.S. market.

The Vanguard ETF isn't the only fund that tracks the S&P 500; its many competitors include the SPDR fund mentioned below. However, the Vanguard fund charges expenses of just 0.03% per year, making it one of the cheapest options.

For those looking to invest in a diversified portfolio of large-cap stocks, Vanguard S&P 500 is a solid choice. It won't beat the market, but it won't disappoint you, either.

2. SPDR S&P 500 ETF

An alternative to the Vanguard S&P 500 ETF is the SPDR S&P 500 ETF (NYSEMKT:SPY). The SPDR fund has exactly the same objective as its Vanguard counterpart, and the two ETFs are very similar in terms of performance.

The SPDR ETF has historical significance as the first major ETF to gain traction in the U.S. market. It's also the largest fund currently, with $333 billion under management. Yet with an expense ratio of 0.09%, it's more expensive than the Vanguard S&P 500 ETF.

SPDR S&P 500 is one of the most heavily traded ETFs in the market, and that gives it a level of liquidity that makes it easy for long-term investors and short-term traders alike to buy and sell shares in large quantities without adversely moving the market. For typical Robinhood investors with small amounts to invest, the extra liquidity isn't terribly important, but the slightly higher fee also isn't that big a deal, amounting to $0.60 a year for every $1,000 invested.

3. ARK Innovation ETF

ARK Innovation ETF (NYSEMKT:ARKK) is a different sort of exchange-traded fund from the S&P 500-trackers above. It's an active ETF run by ARK Invest, with budding investing legend Cathie Wood at the helm. Holdings change daily, with the fund disclosing its purchases and sales to investors after the close of each trading session.

ARK Innovation is ARK Invest's best-of-the-best ETF, incorporating top ideas from each of the fund family's four other actively managed ETFs. With stocks focusing on fintech, genomics, next-generation internet, and industrial innovation, ARK Innovation has soared 181% in the past year -- absolutely crushing the 20% returns of the S&P 500 ETFs.

The ETF charges a higher annual expense ratio of 0.75% to compensate Wood and ARK Invest for their management and other costs. However, for those looking for active management from a proven manager, ARK Innovation has quickly risen to prominence.

4. Direxion Daily S&P Oil & Gas Exploration & Production Bull 2x ETF

The fourth ETF on the Robinhood list is a mouthful. Direxion Daily S&P Oil & Gas Exploration & Production Bull 2x ETF (NYSEMKT:GUSH) is a leveraged ETF, meaning that it tracks an index but provides multiplied daily returns compared to traditional ETFs.

This ETF tracks a group of stocks that are all in the business of exploring for and extracting oil and natural gas. You'll find those same stocks in the traditional SPDR S&P Oil & Gas Exploration & Production ETF (NYSEMKT:XOP), and they include companies like ExxonMobil, Marathon Oil, and Diamondback Energy.

However, the Direxion ETF aims to produce double the daily move of that index. So if oil and gas stocks rise 1% on a given day, this ETF would go up by 2%. The same is true on the downside, with any daily losses magnified as well.

Leveraged ETFs are riskier than regular ETFs, and Direxion has a pricey expense ratio of 1.04%. It's not as good a long-term play as the ETFs above. It's tailored more toward short-term traders expecting to benefit from rising oil prices that, in turn, will boost the stocks of these energy E&P companies.

5. iShares Silver Trust

Last up is iShares Silver Trust (NYSEMKT:SLV). This fund is a commodity ETF tracking the price of silver.

iShares Silver owns almost 20,000 tons of silver bullion, with each of its shares corresponding to just under 0.93 ounces of silver at current levels. However, the ETF charges an annual expense ratio of 0.50%, and because its silver doesn't generate any cash, it takes a portion of the bullion and sells it at regular intervals. That's why the initial target of 1 ounce per share has fallen to 0.93% over the course of 15 years since its 2006 inception.

For investors looking to get exposure to silver, iShares Silver has the benefit of not forcing you to buy and store large bars of metal. Many believe that silver has significant appreciation potential, but unlike traditional stock ETFs, there's no underlying business to generate earnings or pay dividends on iShares Silver's silver holdings.

Robinhood investors are smarter than you think

Robinhood investors get a bad rap, but these ETF holdings are quite astute. The combination of stalwart S&P 500 index funds and some more aggressive plays on specific themes is consistent with a solid asset allocation strategy. If you think that energy and precious metals will do well, then adding an energy ETF and a silver ETF in moderation to a core of S&P index funds and a well-diversified active ETF is a perfectly reasonable way to proceed.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.