The three essential elements of a contract are the offer, the acceptance and the consideration. A contract can contain other elements, but these three are what must be contained in the contract to ensure it is legal and binding.
An offer is the defined promise of one party to be performed for the other listed party. The acceptance element occurs when the second party answers whether or not to accept the offer of the first party.
Consideration is defined as what is done or paid to compensate the promise of the first party. Consideration does not have to be in a monetary form, as it can be a right, interest or other benefit given in exchange for the promise.