Hi khorshed,
Welcome
Here is the answer for your question:
Informal business letters: Examples of informal letters include congratulatory letters, complimentary letters, requests for favors, thank you letters, and letters of encouragement. You may want to consider investing in personalized notecards to give an informal communication that extra personal touch.
Whereas Formal business letters include: complaint letter from business to business, complaint letter from a business customer to a telephone company,a letter to a magazine expressing an opinion about tourism,a letter in application for a job.
I think that helps.
