As per section 2(d) When at the desire of the promisor, the promisee or any other person has done or abstained from doing something or does or abstains from doing something or promises to do or abstain from doing something, such act or abstinence or promise is called a consideration for the promise.

Agreements are all in the following types:
1. Agreement no enforceable by law - Any essential of a valid contract is not available.
2. Agreement enforceable by law - All essentials of a valid contract are available.

(1) A contract in which both the parties commit to perform their respective promises is called a bilateral contract.
(2) For Example: 'R' offers to sell his fiat car to'S' for Rs 10,00,000 on acceptance of 'R's offers by 'S', there is a promise by 'R' to sell the car and there is a promise by'S' to purchase the car, there are two promises.

(i) A void contract is really not a contract at all. But voidable contract is one which is enforceable by law.
(ii) The term void contract is an agreement without any legal effect whereas the term voidable contract agreement with legal effect.
(iii) An agreement with minor is a void contract whereas where A threatens to kill B, if he does not transfer his house in A's favour for a very low price is voidable contract.

1. Revocation can also refer to the termination of an offer.
2. An offeror may revoke an offer before it has been accepted but the revocation must be communicated to the offeree, although not necessarily by the offeror.