Question

what is the format of letter writing plz tell me

Answer 1

which letter formal or informal

Answer 2

Informal letters are written to close acquaintances of the writer, their friends, family, relatives etc. Since they are written to close relations the letters have an informal and personal tone. Casual language is used while writing informal letters. And sometimes the letters may even have an emotional undertone.
Informal letters are mainly used for personal communication. So they do not have to follow any specific pattern, format or conventions. They can be written as per the writer’s wishes and the requirement of the situation. So the letter is written in a personal fashion in casual unassuming language.