A shareholder is anyone who owns at least one share of a company's stock. Owning shares means you own a small piece of the company with rights like voting and receiving dividends.
There are two main types: common shareholders (voting rights but last during liquidation) and preferred shareholders (priority on dividends but usually no vote).
For example, if you own 100 shares of Apple out of roughly 15 billion total shares, you are technically a part-owner — just a very small one.