Usually someone you know or are familiar with e.g friend.
Purpose
To keep in touch with someone you know.
You will give information/enquire about
things that are of mutual interest
Format
You need to include your address and the
date but not their address. The salutation
will be informal e.g. 'Dear Mum' and the
closure will be informal too such as 'Love
from'.
Content
The question will tell you what the
content should be. However, always
plan your letter.