In finance, an option is a contract between a buyer and a seller that gives the buyer of the option the right, but not the obligation, to buy or to sell a specified asset (underlying) on or before the option's expiration time, at an agreed price, the strike price.
In return for granting the option, the seller collects a payment (the premium) from the buyer. Granting the option is also refered to as "selling" or "writing" the option.
*A call option gives the buyer the right to buy the underlying at the strike price.
*A put option gives the buyer of the option the right to sell the underlying at the strike price.