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Letter are of two types-Formal and Informal.Informal letters are letters that could be written to our friends, family, relatives and other near and dear ones. They have a format but that need not be followed strictly. Thus, there is no set pattern decided for informal letters.Formal letters are social or business letters. They follow a formal and strict pattern. Social letters can be friendly letters or notes of invitations. Business letters are usually letters of application, letters written to editors or municipal corporations, etc.