What are the Options? Options Basics : Calls and Puts

An option is a contract written by a seller that conveys to the buyer the right — but not the obligation — to buy (in the case of a call option) or to sell (in the case of a put option) a particular asset, such as a piece of property, or shares of stock or some other underlying security, such as, among others, a futures contract. In return for granting the option, the seller collects a payment (the premium) from the buyer.
For example, buying a call option provides the right to buy a specified quantity of a security at a set agreed amount, known as the 'strike price' at some time on or before expiration, while buying a put option provides the right to sell. Upon the option holder's choice to exercise the option, the party who sold, or wrote, the option must fulfill the terms of the contract.
The theoretical value of an option can be evaluated according to several models. These models, which are developed by quantitative analysts, attempt to predict how the value of the option will change in response to changing conditions. Hence, the risks associated with granting, owning, or trading options may be quantified and managed with a greater degree of precision, perhaps, than with some other investments.
Exchange-traded options form an important class of options which have standardized contract features and trade on public exchanges, facilitating trading among independent parties. Over-the-counter options are traded between private parties, often well-capitalized institutions that have negotiated separate trading and clearing arrangements with each other. Another important class of options, particularly in the U.S., are employee stock options, which are awarded by a company to their employees as a form of incentive compensation.