Friendly letters are the least formal and have a more conversational tone, usually because the writer already has a close relationship with the receiving party. Because of this, they can be written by hand or typed, and there isn't a definite structured format for writing such letters. However, it is best to write a letter in three simple parts. The first part is an introduction where you'll give a brief "teaser" as to what the subject is. If you don't formally know the person you're writing to, introduce yourself at this point. This leads into the main body where you explain every topic you are including in the letter. Finish it with a conclusion where you can summarize what you just wrote and/or thank the recipient for reading.
A business letter format is good to use for most formal letters. Always type these letters on 8X11 paper. You should always start this letter with the date it is written in the top corner followed by the name and title of the recipient and the business and address the letter is being sent to. It also helps to precede this with your own return address. Introduce yourself in the first paragraph and briefly explain your purpose for writing. Specific types of letters have specific details they'll require. A letter for a job application should include your qualifications and why you are applying. A query letter for a book should explain what the book is about and why you think it will sell. Everything should be in a professional, polite and courteous tone, even if it is a letter of complaint. Be clear, concise and to the point in your subject. Give a second summary at the end of the body, possibly explaining how the subject you're writing about benefits them, and finish the letter with a formal closing.
When writing an invitation letter, the most important thing is the event taking place, the time and the location. These must be the first points brought up in the letter. Once the recipient knows what, where and when, you can go into more details about your event, such as who else is invited and what type of dress is recommended. A polite request for an RSVP should be included with the date you need that confirmation by.