Stocks
Stocks are probably what you hear about most often when people talk about the "market." They allow you to invest in a company and each piece of ownership is known as a "share". Each share is worth a certain dollar amount that changes throughout the day as stocks are bought or sold in real time on stock exchanges. In addition, you can also buy a "fractional share" or a small slice of a company's stock rather than the whole share. This can make it easier to start investing since the stock for some companies can trade for hundreds or even thousands of dollars per share.
Mutual Funds
Think of mutual funds as an investment stew. Investments, such as stocks, bonds, and other ingredients are mixed together (some funds may invest only in stocks, or only in bonds) and sold as 1 dish, creating a mutual fund. They offer a way to buy different investments packaged together, or served together like a "dish," and sold together as 1 entity instead of as individual companies. Investments in mutual funds change all the time, as they are managed by a team of professionals who decide which investments to buy and sell. They often come with additional fees (some low, some high) that stocks don’t have because professional managers are making the investment selections. The price of a mutual fund is updated at the end of each business day. You can find out more about each fund's objective and strategies in its prospectus.
ETFs (exchange-traded funds) are another sort of investment stew that mixes together stocks and/or bonds, and sells them for 1 price. They often try to mimic a major stock index, like the S&P 500®, which represents the 500 largest companies in the United States. Since you can't buy from the S&P 500® directly, and may not want to buy stock in each individual company, you can buy one ETF "unit" or "share" and invest in all these companies at once, trading real-time like stocks. And since ETFs contain a mix of assets, they can be a great way to diversify your portfolio and potentially lower your risk and exposure.