The definition of a friendly letter is an informal written correspondence with someone to whom the writer has a relationship. Usually a friendly letter goes to a family member or friend, and the tone is familiar and relaxed.
Friendly letters are used to communicate with someone in a similar manner to having a conversation. The topics typically include telling the reader what is happening in the writer's life. Sometimes, the writer uses a friendly letter to thank the recipient for something along with other personal information. Often the writer asks questions of the reader as well.
There are five parts to a friendly letter: heading, salutation, body, closing and signature. The heading is comprised of the address of the writer and the date, though the address is often omitted. The salutation greets the reader. It says something like "Dear Mary," or "Hi Jane." The salutation is capitalized and followed by a comma. The body is the part of the letter with the information. The closing in a friendly letter says something like "Love," "Your friend" or "Fondly." The first word of the closing is capitalized, and the last word is followed by a comma. The signature is the writer's name and most often just the first name.