ETF's and mutual funds both hold a basket of stocks, bonds, or other assets, so that you can buy that one holding and have exposure to a wider array of investments that are held in the fund or ETF without having to buy each of those individually. For example, an ETF or fund that tracks the S&P 500 will give you exposure to all the stocks held in the S&P without having to buy shares in each individual company on your own.
ETF's trade like stocks and are priced throughout the day; whereas, mutual funds are priced once per day after the markets have closed. So, if you place an order to buy or sell an ETF, you will have a better sense of what you will be paying since you will know the market price when you place the trade; with a mutual fund, you may have a sense of whether the price will be higher or lower after the markets close, but you won't know for sure what price you will pay or receive per share of a mutual fund until after the markets close for the day.
You can also place limit orders for ETFs since they trade like stocks, which you can't do with mutual funds. The expense ratios on ETFs are almost always lower than on a similar mutual fund (e.g. the expenses on a tech sector ETF will tend to be lower than on a tech sector mutual fund).
There are a lot of no-load mutual funds you can choose from to avoid paying a commission (or load); whereas, on ETFs, you may have to pay a commission like you would when you buy/sell a stock. However, there are more and more custodians that are either offering their own ETFs with no trading commissions (e.g. Vanguard, Schwab, etc.) or offering some ETFs with no trading commissions (e.g. Fidelity offers around 70 ETFs commission-free). I think you will continue to see more and more custodians offering commission-free ETFs in some form or fashion.
That being said, even if you have to pay a commission on an ETF, that is usually going to be far less than if you bought a load bearing mutual fund. For example, if you wanted to buy $10,000 of an ETF, you might incur an $8 or $10 commission to buy and an $8 or $10 commission to sell. So, 0.002% maybe on the top end ($20 / $10,000); whereas, if you bought a mutual fund with a 5% front-end load, you would pay $500 just to buy into it, though you generally wouldn't pay anything on the sale. So even if you bought or sold a commission bearing ETF in chunks, your commissions would probably be less than most load-bearing mutual funds.
At the end of the day, ETFs will tend to be able to achieve the same goals/ends as a mutual fund. The main bnefits tend to be the lower expense ratios and the increased flexibility when it comes to trading. If you can find a custodian who offers a wide variety of commission-free ETFs, they are a great way to help build a diversified portfolio.