The purpose of a contract is to establish the agreement which the parties have made and to state their rights and duties as stated in the agreement. For example, when a person offers to sell something for a stated price, this is a promise and it is worth something. When a person accepts the offer by promising to pay for the item when it is delivered, this promise is also worth something. An express contract is an actual agreement between parties. The terms of which are stated openly, at the time the contract is made, in clear and certain language, either spoken or written.
Express contracts can be bilateral or unilateral.
A bilateral contract is one in which each party who has promised something has a duty to do it. For example, in a contract of sale for a car, one party has a duty to deliver the car they are selling, and the other has a duty to pay for the car. A unilateral contract exists when one party has made a promise in return for something other than a returned promise. In these types of contract, only the party that has made the promise has a duty to do what they said, because the other party has already performed their part of the contract. An option is a right which acts as a continuing offer, given for consideration, to purchase or lease property at an agreed upon price and terms, within a specified time.