Volatility may also affect premiums or discounts to net asset values, resulting in higher costs for the investor.

ETFs with higher liquidity can shrink bid/ask spreads since the more interested market makers there are, the closer the highest and lowest offered prices to sell are likely to be. 

Similarly, ETFs with lower liquidity tend to have larger bid/ask spreads.

Investors may pay more for an ETF than the value of its underlying stocks or bonds (a premium). Conversely, investors may sell an ETF for less than the value of its holdings (a discount).

On top of commissions, investors also pay the "spread" when buying or selling ETFs, the difference between the higher price you pay to acquire a security and the lower price at which you can sell it.

Some brokerages charge you a commission when you buy or sell an ETF. 

However, Schwab doesn't charge commissions for online ETF trades.¹
 

Due to typically lower turnover and the in-kind creation/redemption process, ETFs typically pass through fewer capital gains to U.S. investors, including expatriates.

Most ETFs disclose their holdings on a daily basis.

ETFs are very versatile, letting you easily move money between specific asset classes, like stocks, bonds, or commodities.