Shares of stock are sold by a company to raise money to start or expand business. They literally are a share of the ownership, although that is usually just a tiny fraction of ownership. Shares are later traded on the stock market (sold from person to person) and their price increases or decreases on the basis of increased or decreased earnings in the company which increases or decreases the value of each share of stock. Or they increase or decrease in value on the expectation that the shares will be earning more or earning less in the future than they are now.

A public company that is traded on the stock exchange, issues shares of stock. Each share represents a fractional ownership of the company. For example, if a company issued 100 shares of stock at $20 each, they would have collected 100 x $20 = $2000. If you purchased 1 share at $20 you would own 1% of the company. Of course companies don't just issue 100 shares, they issue thousands, sometimes millions. Once the shares begin trading on the stock exchange, people place bids to buy shares opening that something is willing to sell them shares at their bid price. If no one does, and they still want shares they will keep raising their bid price until someone agrees to sell their shares, that is how stock prices go up. The reverse can happen as well, if you want to sell your shares, you "ask" a specific price for your shares, if no one buys the shares and you still want to sell you might keep lowering your price until someone agrees to buy them from you. That is how prices go up and down on the stock market

I am 14 years old and my father has bought me 50 shares in a stock, each worth about 20 dollars each. I don't quite understand the concept of shares and the stock, so can someone please explain them to me in a clear explanation?