The form and content of an open letter depends on its purpose. If the goal is to inform the public about something that happened, it should specify what took place. For example, a letter to a newspaper about discriminatory treatment a person received at a restaurant should include the name of the restaurant, the date of the incident, the details of what transpired and how it affected the person involved.
Some people write open letters to express opinions. These letters are typically less specific, providing an overview of one or more issues and how the author feels about them. An open letter to the President of the United States, for example, might summarize the writer's thoughts on the economy or foreign policy and make suggestions about how the author believes the president should handle them. Typically, people send this type of letter to a newspaper or magazine for publication on the opinions and editorials page. "J'accuse!," a famous letter written in 1898 by writer Emil Zola to then French president Félix Faure about the imprisonment of Alfred Dreyfus, is an example of this type of political open letter.
Other open letters are very personal, such as those expressing love or animosity toward a mother, father, sibling or spouse. These letters are often published on the Internet on a personal blog or website or on an editorial page that is normally written by someone else. To avoid lawsuits for libel and slander as well as future embarrassment, it's a good idea to use a pseudonym to identify all parties when writing such a letter.