Importance of business letter
It is used to communicate ideas in a formal way between people and/or companies on a professional level.
Meaning
Business letters are formal letters used for business-to-business, business-to-client, or client-to-business correspondence.

parts
Date & Sender's Address * The first line of a business letter should be the date the letter was written or completed. Directly underneath the date is the sender's address. Do not include the sender's name here. Sometimes the sender's address is listed on a letterhead, in which case the address should not be repeated under the date.
Recipient's Address * If known, the recipient's address should include the name of the person to whom the letter is directed. You should also include a title (Mr., Mrs., Ms., or Dr.) in front of the name. If you are including the country with the address, capitalize the country name.
Salutation
* The salutation should be the same as the name written with the recipient's address, followed by a colon. It is fine to only use the first name if you personally know the person and you typically refer to her by her first name. If you do not know the recipient's name, it is fine to use the salutation, "To Whom it May Concern."
Body
* The body of the letter should be formally written. Use single-spaced lines, except between paragraphs, where a double-spaced line should be inserted. The closing paragraph should summarize what was previously stated throughout the letter.
Closing
* To close the letter, insert a phrase such as "Thank you" or "Best regards," followed by a comma. Insert four lines between the closing and sender's name. This space will be used for your signature once the letter is printed.

kinds

1. Acknowledgement Letter : This type of letter is written when you want to acknowledge some one for his help or support when you were