Stocks (also capital stock, or sometimes interchangeably, shares) consist of all the shares by which ownership of a corporation or company is divided. A single share of the stock means fractional ownership of the corporation in proportion to the total number of shares. This typically entitles the shareholder (stockholder) to that fraction of the company's earnings, proceeds from liquidation of assets (after discharge of all senior claims such as secured and unsecured debt), or voting power, often dividing these up in proportion to the number of like shares each stockholder owns. Not all stock is necessarily equal, as certain classes of stock may be issued, for example, without voting rights, with enhanced voting rights, or with a certain priority to receive profits or liquidation proceeds before or after other classes of shareholders.

Stock can be bought and sold privately or on stock exchanges. Transactions of the former are closely overseen by governments and regulatory bodies to prevent fraud, protect investors, and benefit the larger economy. As new shares are issued by a company, the ownership and rights of existing shareholders are diluted in return for cash to sustain or grow the business. Companies can also buy back stock, which often lets investors recoup the initial investment plus capital gains from subsequent rises in stock price. Stock options issued by many companies as part of employee compensation do not represent ownership, but represent the right to buy ownership at a future time at a specified price. This would represent a windfall to the employees if the option were exercised when the market price is higher than the promised price, since if they immediately sold the stock they would keep the difference (minus taxes).

Stock bought and sold in private markets fall within the private equity realm of finance.

A person who owns a percentage of the stock has the ownership of the corporation proportional to their share. The shares form a stock; the stock of a corporation is partitioned into shares, the total of which are stated at the time of business formation. Additional shares may subsequently be authorized by the existing shareholders and issued by the company. In some jurisdictions, each share of stock has a certain declared par value, which is a nominal accounting value used to represent the equity on the balance sheet of the corporation. In other jurisdictions, however, shares of stock may be issued without associated par value.

Shares represent a fraction of ownership in a business. A business may declare different types (or classes) of shares, each having distinctive ownership rules, privileges, or share values. Ownership of shares may be documented by issuance of a stock certificate. A stock certificate is a legal document that specifies the number of shares owned by the shareholder, and other specifics of the shares, such as the par value, if any, or the class of the shares.

In the United Kingdom, Republic of Ireland, South Africa, and Australia, stock can also refer, less commonly, to all kinds of marketable securities.

Stock typically takes the form of shares of either common stock or preferred stock. As a unit of ownership, common stock typically carries voting rights that can be exercised in corporate decisions. Preferred stock differs from common stock in that it typically does not carry voting rights but is legally entitled to receive a certain level of dividend payments before any dividends can be issued to other shareholders.^{[page needed]} Convertible preferred stock is preferred stock that includes the ability of the holder to convert the preferred shares into a fixed number of common shares, usually any time after a predetermined date. Shares of such stock are called "convertible preferred shares" (or "convertible preference shares" in the UK).

New equity issue may have specific legal clauses attached that differentiate them from previous issues of the issuer. Some shares of common stock may be issued without the typical voting rights, for instance, or some shares may have special rights unique to them and issued only to certain parties. Often, new issues that have not been registered with a securities governing body may be restricted from resale for certain periods of time.