Think of an open letter like the letter to the editor's funky cousin. You are still writing a letter that is intended to be read by the general public. You are still expressing an opinion. A letter to the editor does those things too. The difference is this- when you are writing an open letter, it is addressed to a specific audience. This may be a specific person, or group of people or even, very occasionally, an inanimate object or idea. For example, famous open letters have been written to political leaders, to civil organizations, to graduating classes, to offspring, to family, to days of the week, and even to anthropomorphized ideals.
Why does addressing your letter to a specific audience, make the open letter different? Where is the power in that? Simply speaking, though the open letter is being written with the knowledge that everyone can read it, you are also writing to one audience. The public takes that knowledge as into account when they read. If you write to denounce an act of a political figure, they know of their act. If you write to give advice to a specific person, the public can infer that person's situation and take the advice if their situation is similar. Your arguments are understood to be framed and crafted to counter those of whom you addressed. Even if you choose to write to a Monday Morning, people know instantly what is the mood you are trying to convey (who, after all, doesn't have Monday Mornings now and then?)
By writing to a specific person, you choose to make it more personal and can write in a more intimate style than a letter to an editor might allow you to. The person(s) you address are known to you- how intimate relationship determines the overall tone of your letter. A writer might make an open letter to his daughter, giving advice about growing up. That will be a more lovingly letter with references to specific incidences. The tone will contrast greatly when compared to Martin Luther King Jr.'s open letter on civil rights, written from a jail cell. As it should. The person you write to is also usually an indication of your letter's purpose. You might write an open letter to give advice; to start or end a dialogue about a certain idea, especially social movements; you may be urging a change in thinking, or a change in policy; you may be warning people of dangers you foresee; or you may be merely trying to make them laugh.
In format, an open letter will start with the salutation to the person you addressed the letter to. Much of the rest of the letter will follow an essay format, and your purpose will dictate the level of formality in the open letter. Rather than merely ending the essay, you still sign off with a benediction appropriate to whom you are addressed the letter to (in more cases, "sincerely" works fine).
Have you ever read an open letter, or tried to write one? In this month when so many conversations are going about the civil rights movement, and with International Women's Day coming up in March , it makes me wonder what open letters will we be seeing in the weeks to come?
Open Letter to Open Letter Writers in 2014
Open Letter...About Open Letters
Wikipedia's entry on Open Letters
The Art of the Open Letter