In order to have an Agreement, there must be an offer, acceptance of the offer, and consideration paid by both parties.  Contracts are entered into all the time without persons realizing their conduct resulted in a contract, which is a binding legal obligation that is enforceable.

An agreement can be verbal or written.

Products and/or services advertised in a store, on a website, or by other means are invitations to enter into a contract.  An act or statement for purchase of the product or service is acceptance.  A contract is formed when money is paid for the product or service.  For example,  advertisements for sale of computers in a computer store or on a website is an offer to buy a computer.  Handing a person the amount of money advertised for the computer is acceptance of the offer.  Accepting the money and giving the computer in exchange is consideration on both sides that results in a written contract, that is evidenced by the receipt.

An example of a service contract is