ETFs are a type of investment, not a specific choice. In other words, there are good ETFs and bad. What you see is the general statement that ETFs are preferable to most mutual funds, if only for the fact that they are low cost. An index ETF such as SPY (which reflects the S&P 500 index) has a .09% annual expense, vs a mutual fund which average a full percent or more.
sheegaon isn't wrong, I just have a different spin to offer you. Given a long term return of say even 8% (note - this question is not a debate of the long term return, and I purposely chose a low number compared to the long term average, closer to 10%) and the current CD rate of <1%, a 1% hit for the commission on the buy side doesn't bother me. The sell won't occur for a long time, and $8 on a $10K sale is no big deal. I'd not expect you to save $1K/yr in cash/CDs for the years it would take to make that $8 fee look tiny. Not when over time the growth will overshaddow this.
One day you will be in a position where the swings in the market will produce the random increase or decrease to your net worth in the $10s of thousands. Do you know why you won't lose a night's sleep over this? Because when you invested your first $1K, and started to pay attention to the market, you saw how some days had swings of 3 or 4%, and you built up an immunity to the day to day noise. You stayed invested and as you gained wealth, you stuck to the right rebalancing each year, so a market crash which took others down by 30%, only impacted you by 15-20, and you were ready for the next move to the upside. And you also saw that since mutual funds with their 1% fees never beat the index over time, you were happy to say you lagged the S&P by .09%, or 1% over 11 year's time vs those whose funds had some great years, but lost it all in the bad years.
And by the way, right until you are in the 25% bracket, Roth is the way to go. When you are at 25%, that's the time to use pre-tax accounts to get just below the cuttoff.
Last, welcome to SE.
Edit - see sheegaon's answer below. I agree, I missed the cost of the bid/ask spread. Going with the lowest cost (index) funds may make better sense for you. To clarify, Sheehan points out that ETFs trade like a stock, a commission, and a bid/ask, both add to transaction cost. So, agreeing this is the case, an indexed-based mutual fund can provide the best of possible options. Reflecting the S&P (for example) less a small anual expense, .1% or less.