Communication through exchange of letters is known as correspondence. A Businessman  writes and receivesletters in his day to-day transactions, which may be called business correspondence.
Business correspondence or business letter is a written communication between two parties. Business letters are formal letters used for business-to-business, business-to-client, or client-to-business correspondence. A means through which views are expressed and ideas or information is communicated in writing in the process of business activities.means it is a partnership of two person to improve their letter.
Businessmen may write letters to supplier of goods and also receive letters from the suppliers. Customers may write letters to businessmen seeking information about availability of goods, price, quality, sample etc. or place order for purchase of goods. Thus,business letters may be defined as a media or means through which views are expressed and ideas or information is communicated in writing in the process of business activities.
The statements written in the letter should be accurate to, the best of the sender’s knowledge. Accuracy demands that there are no errors in the usage of language - in grammar, spellings, punctuations etc. An accurate letter is always appreciated. The letter should contain only essential information. A complete letter is one that provides all necessary information to the users.
The quality will be maintained if we give proper attention to each and every part of the letter. 
The essential parts of a business letter are as follows:
Heading,  Date,  Reference,  Inside address, Subject,  Salutation, Body of the letter,  Complimentary close,  Signature, Enclosures,Copy circulation ,Post script.