When you buy a share, you buy a fraction of a company. Not a promise from the company, and not a deposit with it. You own a piece of the company itself. That distinction explains nearly everything else about how shares behave.
What ownership entitles you to
As a shareholder you generally have a claim on the company's profits, usually paid as a dividend when the board decides to distribute them, and a vote in certain company decisions in proportion to your holding. You also have a claim on whatever is left if the company is wound up, though that claim ranks behind everyone the company owes money to, which in practice often means nothing is left.
What it does not entitle you to
It does not entitle you to your money back. There is no maturity date and nobody is obliged to buy your shares from you. If you want to convert them into cash, you must find a buyer at a price you are willing to accept. This is why a functioning exchange matters so much: it is the mechanism that makes finding that buyer possible.
It also does not entitle you to a dividend. Dividends are declared at the discretion of the board. A company can be profitable and pay nothing, choosing to reinvest instead.
Why the price moves
A share's price is whatever the next buyer will pay. That number reflects a collective, constantly revised guess about the company's future earnings, and guesses change as new information arrives. Strong results, a new competitor, a change in regulation, or a shift in the general level of interest rates will all move the price, sometimes sharply.
The risk, stated plainly
You can lose money. If the company performs poorly, or if sentiment turns, the price can fall below what you paid and stay there. In the worst case, if the company becomes insolvent, shares can become worthless and shareholders are last in the queue. This is not a remote scenario dressed up as a disclaimer; it is the ordinary risk of equity ownership, and it is the reason shares have historically offered higher returns than deposits.
Only invest money you can afford to leave invested through a downturn, and do not concentrate everything you have in a single company.
This is education, not advice
This guide explains general concepts. It does not take account of your objectives or financial circumstances and is not a recommendation to buy or sell anything. If you are unsure, seek independent advice.