All letters, whether business or otherwise can be classified as formal and informal or official, demi-official and personal. These classifications are conveniences that make the study of the subject easier. In the previous decades, school children were assiduously (constantly) taught to write formal and informal letters.
Business letters for the most part can be termed formal or official, though they can be informal. Informal letters give a certain freedom to the writer. His style can be conversational or chatty, whereas a business letter has to follow a certain protocol in its structure and a certain level of formality in its style. It does not mean that the style should be frigid and the tone curt. Since it is often times addressed to people not previously known or familiar undue liberties should not be taken. The style should be easy and natural. Since there is a lot of similarity between oral and written communications, try to write as you will speak. The whole secret of a good letter is that it speaks on behalf of the sender.
As we have already seen, a business letter represents the individual and the organization. A badly written letter will bring discredit to the organization and destroy confidence and the goodwill of the recipients of the letter.