A stock is a security with which you buy a small part of a publicly traded company. For example, if you own a stock of Apple, a large bank or a car manufacturer, you own a part of this company. In the past, as a stockholder, you would have received a certificate from this company in exchange for your money. Today, it's virtual, but still based on the same principle: a certain amount of money can be exchanged for stocks, and with that, a part of that company.
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