In business and finance, a share (also referred to as equity share) of stock means a share of ownership in a corporation (company). In the plural, stocks is often used as a synonym for shares especially in the United States, but it is less commonly used that way outside of North America.

In the United Kingdom, South Africa, and Australia, stock can also refer to completely different financial instruments such as government bonds or, less commonly, to all kinds of marketable securities
Types of stock
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.Convertible preferred stock is preferred stock that includes an option for the holder to convert the preferred shares into a fixed number of common shares, usually anytime after a predetermined date. Shares of such stock are called “convertible preferred shares” (or “convertible preference shares” in the UK)

Although there is a great deal of commonality between the stocks of different companies, each new equity issue can have legal clauses attached to it that make it dynamically different from the more general cases. Some shares of common stock may be issued without the typical voting rights being included, for instance, or some shares may have special rights unique to them and issued only to certain parties. Note that not all equity shares are the same.

Stock derivatives
A stock derivative is any financial instrument which has a value that is dependent on the price of the underlying stock. Futures and options are the main types of derivatives on stocks. The underlying security may be a stock index or an individual firm’s stock, e.g. single-stock futures.

Stock futures are contracts where the buyer is long, i.e., takes on the obligation to buy on the contract maturity date, and the seller is short, i.e., takes on the obligation to sell. Stock index futures are generally not delivered in the usual manner, but by cash settlement.

A stock option is a class of option. Specifically, a call option is the right (not obligation) to buy stock in the future at a fixed price and a put option is the right (not obligation) to sell stock in the future at a fixed price. Thus, the value of a stock option changes in reaction to the underlying stock of which it is a derivative. The most popular method of valuing stock options is the Black Scholes model. Apart from call options granted to employees, most stock options are transferable
Trading
A stock exchange is an organization that provides a marketplace for either physical or virtual trading shares, bonds and warrants and other financial products where investors (represented by stock brokers) may buy and sell shares of a wide range of companies. A company will usually list its shares by meeting and maintaining the listing requirements of a particular stock exchange. In the United States, through the inter-market quotation system, stocks listed on one exchange can also be bought or sold on several other exchanges, including relatively new so-called ECNs (Electronic Communication Networks like Archipelago or Instinet).

In the USA stocks used to be broadly grouped into NYSE-listed and NASDAQ-listed stocks. Until a few years ago there was a law that NYSE listed stocks were not allowed to be listed on the NASDAQ or vice versa.

Many large non-U.S companies choose to list on a U.S. exchange as well as an exchange in their home country in order to broaden their investor base. These companies have then to ship a certain number of shares to a bank in the US (a certain percentage of their principal) and put it in the safe of the bank. Then the bank where they deposited the shares can issue a certain number of so-called American Depositary Shares, short ADS (singular). If someone buys now a certain number of ADSs the bank where the shares are deposited issues an American Depository Receipt (ADR) for the buyer of the ADSs.

Likewise, many large U.S. companies list themselves at foreign exchanges to raise capital abroad.
Arbitrage trading
Although it makes sense for some companies to raise capital by offering stock on more than one exchange, a keen investor with access to information about such discrepancies could invest in expectation of their eventual convergence, known as an arbitrage trade. In today’s era of electronic trading, these discrepancies, if they exist, are both shorter-lived and more quickly acted upon. As such, arbitrage opportunities disappear quickly due to the efficient nature of the market.
Buying
There are various methods of buying and financing stocks. The most common means is through a stock broker. Whether they are a full service or discount broker, they arrange the transfer of stock from a seller to a buyer. Most trades are actually done through brokers listed with a stock exchange, such as the New York Stock Exchange.

There are many different stock brokers from which to choose, such as full service brokers or discount brokers. The full service brokers usually charge more per trade, but give investment advice or more personal service; the discount brokers offer little or no investment advice but charge less for trades. Another type of broker would be a bank or credit union that may have a deal set up with either a full service or discount broker.

There are other ways of buying stock besides through a broker. One way is directly from the company itself. If at least one share is owned, most companies will allow the purchase of shares directly from the company through their investor relations departments. However, the initial share of stock in the company will have to be obtained through a regular stock broker. Another way to buy stock in companies is through Direct Public Offerings which are usually sold by the company itself. A direct public offering is an initial public offering in which the stock is purchased directly from the company, usually without the aid of brokers.

When it comes to financing a purchase of stocks there are two ways: purchasing stock with money that is currently in the buyer’s ownership, or by buying stock on margin. Buying stock on margin means buying stock with money borrowed against the stocks in the same account. These stocks, or collateral, guarantee that the buyer can repay the loan; otherwise, the stockbroker has the right to sell the stock (collateral) to repay the borrowed money. He can sell if the share price drops below the margin requirement, at least 50% of the value of the stocks in the account. Buying on margin works the same way as borrowing money to buy a car or a house, using the car or house as collateral. Moreover, borrowing is not free; the broker usually charges 8-10% interest.

Selling
Selling stock is procedurally similar to buying stock. Generally, the investor wants to buy low and sell high, if not in that order (short selling); although a number of reasons may induce an investor to sell at a loss, e.g., to avoid further loss.

As with buying a stock, there is a transaction fee for the broker’s efforts in arranging the transfer of stock from a seller to a buyer. This fee can be high or low depending on which type of brokerage, full service or discount, handles the transaction.

After the transaction has been made, the seller is then entitled to all of the money. An important part of selling is keeping track of the earnings. Importantly, on selling the stock, in jurisdictions that have them, capital gains taxes will have to be paid on the additional proceeds, if any, that are in excess of the cost basis.