A stock, also referred to as a share, is commonly a share of ownership in a company.
The owners and financial backers of a company may want additional capital to invest in new projects within the company. If they were to sell the company it would represent a loss of control over the company.
Alternatively, by selling shares, they can sell part or all of the company to many part-owners. The purchase of one share entitles the owner of that share to literally a share in the ownership of the company, including the right to a fraction of the assets of the company, a fraction of the decision-making power, and potentially a fraction of the profits, which the company may issue as dividends. However, the original owners of the company often still have control of the company, and can use the money paid for the shares to grow the company.
In the common case, where there are thousands of shareholders, it is impractical to have all of them making the daily decisions required in the running of a company. Thus, the shareholders will use their shares as votes in the election of members of the board of directors of the company. However, the choices are usually nominated by insiders or the board of the directors themselves, which over time has led to most of the top executives being on each other's boards. Each share constitutes one vote (except in a co-operative society where every member gets one vote regardless of the number of shares they hold). Thus, if one shareholder owns more than half the shares, they can out-vote everyone else, and thus have control of the company.