When do you write informal letters?

Formal Letters are used for business matters. You write one to:

Apply for a job
Ask for information
Make an Inquiry
Order goods
Make a complaint

Formal letters should...

Be precise
Have correct grammar
Have correct punctuation
Have a formal tone
Be clear and to the point

Useful Words and phrases for formal letters

You can start the letter using a term such as 
    "Dear Sir"

"With reference to..." 
    "In reply to your letter" 
    "I would like to request a..." 
    "Please could you send me..."
    "I am writing to ask for..." 
    "I would be grateful..." 
    "I enclose a stamped addressed envelope"

Finish the letter using endings such as
    "I look forward to hearing from you..."
    "Yours sincerely"
    "Yours faithfully"

Sample for a complaint letter

Sample for a job application (cover letter)