This one is pretty boring but pretty important to, so I'm going to let my dad ghost write it.
Even though ETFs were developed 69 years after mutual funds, they are quickly becoming a more sought after investment vehicle. Mutual Funds are still much larger in terms of assets largely because most 401(k)s do not allow participants to hold ETFs. Below is a chart allowing you to compare and contrast the differences between ETFs and Mutual Funds:
If the chart is hard to read, the basics are that ETFs are traded like stocks and offer access to a basket of similar stocks for a low fee. Mutual Funds also offer access to a basket of stocks but they are less liquid and cost more.