called formal letters and informal letters.
Formal letters are letters which are sent to people you don’t know. They can be sent for different reasons, for example to complain about something, to give or ask for information or to make an appointment. Formal letters are usually set out in a certain way.
Informal letters are letters which are sent to people you know. For example, sending a postcard telling someone about your holiday, to say, “thank you” for a birthday present or writing to friends or family who might live a long way from you.