In their simplest form, stock options are a contract between two parties that expires at an agreed-upon time in the future. The contract purchaser is buying the right, but not the obligation, to buy (a "call" option) or sell (a "put" option) an asset (the "underlying") at a specific price, on or before the agreed-upon date. The contract seller is accepting the obligation to take the other side of the transaction.
The earliest known options trade dates from 7th century BCE. Thales of Miletus speculated that the year's olive harvest would be especially bountiful, and put a deposit on every olive press in his region of Greece. The harvest was huge, demand for olive presses skyrocketed, and Thales sold his rights, or options, to the presses at substantial profit. The modern history of options trading begins with the 1973 establishment of the Chicago Board Options Exchange (CBOE) and the development of the Black-Scholes option pricing model.
Stock options are defined by several key characteristics. The expiration date specifies when the option contract becomes null and void. The underlying is the asset upon which the stock option is based. The strike price, or exercise price, is the price at which the underlying asset will be bought or sold should the option holder decide to exercise their right to buy or sell. European-style options are only exercisable on the expiration date; American-style options are exercisable at any time before the expiration date.
An ATM, or at-the-money option, is one where the strike price is roughly the same as the current underlying price. An OTM, or out-of-the-money option, is one where the underlying price is far enough away from the strike price that there is no incentive for the holder to exercise the contract. Conversely, an ITM, or in-the-money option, is one where the holder can exercise the option profitably.
The simplest stock options trading strategy is to buy an OTM call (or put) option if the expectation is for a dramatic increase (or decrease) in the price of the underlying. Spreads involve buying one option and selling another; they are often used to lower the initial cost of the position at the expense of lower maximum potential profit. Examples of spreads are verticals, backspreads, bull and bear spreads, ratio spreads, butterflies, and condors.
Stock options allow speculators to make bets on market movement without having to pick an up or down direction. For example, buying both an ATM put and an ATM call would give the holder exposure to a dramatic move in either direction. Because of this, options traders are often said to be trading volatility rather than price.