contract

Please search in google with those keywords and you can find in detail those essentials and with latest case laws and case studies. In simple sentence identity of minds on promise/commitment between sound persons for consideration and for a  possible, legal, promise of a doing one thing or not doing a thing is a contract..

instead of  use the Seach Engine in computer, buy a book and read... it will help you 

1st we will understand what  contract is basically contract is agreement between various parties framed and validated by the indian contract act 

Indian contract act  - under Section 2 of Indian contract act 

1 section 2(a) there is a brief explanation of offer which states that 

(a) when one person signifies to another his willingness to do or to abstain from doing anything , with a view to obtaining the assent of that other to such act or abstinence, he is said to make a proposal.

2 After offer there shoud be acceptance which is explained under section 2(b) which states that 

(b) when the person to who the proposal is made signifies his assent thereto,the proposal is said to be accepted.A proposal, when accepted, becames a promise.

3 Intention to create a legal relationship: There must be a clear intention among the parties that the agreement should be attached by legal consequences and create a legal obligation.

Agreements of a social or domestic nature do not contemplate a legal relationship, and as such, they do not give rise to a contract.

4 There should be a lawful consideration which is explained in the Section 2(d) in The Indian Contract Act, 1872