what is the major difference between a formal and informal letter.

From Internet:

Formal writing is that form of writing which is used for the business,legal,academic or professional purpose. ...
Formal writing must use a professional tone,whereas a personal and emotional tone can be found in informal writing.
In formal writing,use of slang is not at all common,whereas we normally use slang in informal writing.

At school my aunty asked that question,

A. a boy said is: a formal letter is written to people that we aren't familiar with. informal letter are written to people we know.
B. a boy said it isn't true, he said: a formal letter has two addresses while an informal has one address.
pls which one is true (A ) or( B )

I'd go with A.

B is just strange!

The major difference between a formal and informal letter lies in their purpose, tone, language, structure, and formatting.

1. Purpose: A formal letter is usually written for official or business purposes, such as job applications, inquiries, complaints, or business correspondence. On the other hand, an informal letter is more personal and is often written to friends, family members, or acquaintances to share news, express emotions, or have a casual conversation.

2. Tone and Language: In a formal letter, the tone is professional, polite, and respectful. It requires the use of formal language, standard grammar and syntax, and a professional vocabulary. Conversely, an informal letter has a casual, friendly tone and allows for more expressive language, use of contractions, and colloquialisms.

3. Structure: A formal letter typically follows a specific structure, including a formal salutation, introduction, body paragraphs organized in a logical manner, and a formal closing. It often includes titles, such as 'Dear Mr./Ms.' and ends with 'Yours sincerely,' or 'Yours faithfully.' Alternatively, an informal letter has a more relaxed structure, with a casual salutation (e.g., 'Dear [Name],' 'Hi,' or 'Hello'), a conversational style of writing, and a less rigid closing (e.g., 'Best regards,' 'Take care,' or 'Love,').

4. Formatting: Formal letters adhere to specific formatting rules, such as using a block format with the sender's address, date, recipient's address, and a formal heading. They may also include references, bullet points, or numbered lists. In contrast, informal letters are less concerned with formatting rules and may have a more free-flowing structure.

To distinguish between a formal and informal letter, consider the purpose, the intended audience, the level of formality required, and follow the appropriate tone, language, structure, and formatting guidelines for each type.