As per the Indian Contract Act, 1872, a "contract" is an agreement enforceable by law. The agreements not enforceable by law are not contracts. An "agreement" means 'a promise or a set of promises' forming consideration for each other. A promise arises when a proposal.is accepted. By implication, an agreement is an accepted proposal. In other words, an agreement consists of an ' offer' and its 'acceptance'.
Agreement = offer/Proposal + Acceptance
Contract = Agreement + Enforceability by law.

The Legal rules of offer are:
(i) An offer must be are intended to give rise to legal consequences.
(ii) An offer must be intended to create legal relations.
(iii) The terms of an offer must be certain or capable of being made certain.
(iv) An offer must be distinguished from an invitation to offer or quotation.
(v) A general offer must be distinguished from a sped fie offer.
(vi) A general offer must be distinguished from a standing offer or tender.
(vii) Offer must be communicated.
(viii) Attention of the offeree must be drawn to the terms and conditions of the offer.

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.

(i) A voidable contract is one which is enforceable by law at the option of one or more of the parties thereto, but not, at the option of other or others [Section 2 (i)]. .
(ii) A contract is voidable when one of the parties to the contract has not exercised his free consent.
(iii) Examples of voidable contracts are those which are induced by coercion undue influence, fraud, or misrepresentation.
(iv) In all these cases the person whose consent was not freely given may avoid the contract.
(v) Thus, where 'A' threatens to kill 'B' if he does not transfer his house in 'A's favour for a very low price, the agreement is voidable for being the result of coercion.

Every promise and set of promises forming consideration for each other is an agreement. In short, Agreement = Offer + Acceptance