Exchanged-traded funds (ETFs) have become stock market darlings. There are now thousands on the market and more are constantly being added. Not only do professional money managers use ETFs, retail investors use them too. Their simplicity and low fees make them perfect for retirement accounts.

If you're considering using ETFs for your retirement accounts, however, you should know how they work. Not all of what you read is true. Here are a few common misconceptions.

Fees Are Not Always Lower for ETFs

Your 401(k), IRA, and other retirement accounts likely have mutual funds as the only options, but that's changing. Many 401(k) managers are adding ETFs as options alongside mutual funds. Often, these ETFs charge less in fees than mutual funds, but not always.

Sure, the Vanguard Total Stock Market ETF has an expense ratio of only 0.03%. But, the Teucrium Sugar ETF has an expense ratio of 2.93%.

Of the more than 2,000 on the market, only a small portion of those have fees over 1%. Investors should never assume that an ETF is cheaper than a mutual fund.

If you have the option of ETFs in your retirement accounts, look closely at the fees. The funds might be cheaper than mutual funds, but not always.

Tracking Isn't an Exact Science

There has been a push to load retirement funds with low-cost index funds either in the form of ETFs or mutual funds. Rather than trying to beat the market, performing with the market produces better gains over time.

If you are tempted to use an ETF that follows an index, understand that it won't follow the index exactly.

Look at the most popular ETF as an example. The SPDR S&P 500 (SPY) is designed to mirror the S&P 500 index. Although it is close, it's not exact. That's in part because of the fees and expenses. Once expenses are subtracted, the fund may slightly underperform the index. Higher-cost ETFs may diverge more drastically from the index they're designed to mirror.

Look at the ETF's yearly performance alongside the performance of the index. They should be nearly identical.

Not All ETFs are Passively Managed

You might have heard that ETFs are perfect for your retirement portfolio because they're passively managed and that keeps fees lower.

Mutual funds have gained a lot of bad press because of the high fees that come with active management. Not only do the investors in the fund have to pay the manager, they also have to pay taxes and trading costs associated with the activities of the fund.

Passively managed funds have lower expenses because they do not have to employ a team to constantly research and trade in the fund. That's why the fees are lower.

Because passively managed ETFs have these lower fees, they're best for retirement funds since fees can severely erode the gains in a long term retirement fund.

But be careful. There are both actively and passively managed mutual funds and ETFs.

Index ETFs Aren't Identical

Some index ETFs, such as the SPDR S&P 500, follow their index by investing in all of the stocks available to them. Others, such as the Vanguard Total Stock Market ETF, invest in a representative sample of the stocks listed in select indices. The VTI attempts to mirror the total stock market.

It's important is to read about each ETF before committing money. Different strategies will produce different results.

Look at how each index ETF works and compare its performance to the index it follows. Compare the fees they charge, too.

Owning an ETF Doesn't Make You Diversified

Safety is just as important as growth in a retirement portfolio. One of the easiest ways to find safety is by diversifying. You should have a variety of investments in different sectors of the economy.

If you own an ETF that mirrors one of the major indices like the Dow Jones industrial average or the S&P 500, you are well diversified.

If you own a sector ETF, such as the Financial Select Sector SPDR, that is only comprised of financial stocks, you're only diversified in that sector. Most portfolio managers do not consider that to be well-diversified.

ETFs range from covering the entire stock market to only investing in small sectors. Since short-term trading in your retirement account is generally not advisable, make your core holdings ETFs that follow the larger market indices.

Don't attempt to pick the sector that will outperform the rest of the market. That strategy isn't likely to produce higher-than-average gains.