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.

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.

(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.