A friendly letter is one that is addressed to family, friends, classmates, acquaintances, or anybody with whom you have a relationship. Because there are no clear and fast rules for writing friendly letters, they fall under the category of informal writing. You can be as detailed or general in your correspondence as you like; it all depends on how you feel about the person receiving the letter. In English, "friendly" means having or showing goodwill toward someone or something. Your friend is someone who shares your interests and activities; someone with whom you have a friendly relationship. In letters, a friendly tone is one that shows respect to the recipient. It can be done by being formal or not, but it always includes positive aspects of the person being written to. For example, you could say something like this: "I am writing to thank you for giving me such an excellent tour of the museum last week. The information you gave me was very helpful." Here, the writer is being friendly because she is showing that she is grateful to the recipient for giving her useful information. In business letters, being friendly is also important because it creates a good first impression. If you write a business letter without any regard for its formality or tone, then you should expect a similar response from the recipient. That would be incorrect because a business letter must be written in a formal manner if you want the reader to understand it fully.