What are the kinds of formal and informal letters?

Question

in all kinds of letters which of them are formal and informal

Answer 1

I'm not sure I understand the question.  In most cases, if you take the time to write a letter, it should be formal and sincere.  If you are handwriting a note it can be casual, but you still need to use proper spelling, punctuation, and avoid using slang terms.  

The best rule of thumb is to write formally in all instances.  No one will ever complain that your letter is too formal, but some may privately notice if it is too informal.