Letters serve many different purposes. They are broadly classified into two categories: social letters and business letters.
Friendly letters and invitation letters are social letters. Business letters include letters of application, official letters and letters to newspapers.
Letters to your close friends and relatives should be written in an easy, conversational style. They are not much different from a friendly chat. In a friendly letter, you can discuss any subject in any order you like. You can also use idiomatic or colloquial expressions. These are usually considered inappropriate in formal or business letters.
However, this doesn’t mean that you can be careless when writing an informal letter. Sending a badly-written, ill-composed letter to a friend is like insulting them.
Also you need to follow the rules of grammar, spelling, and punctuation in both formal and informal letters. Ungrammatical expressions should have no place in your letters.