A letter is a certain kind of writing. It is usually intended to communicate something to someone else. The writer of the letter writes the letter on paper and sends it to the recipient, the addressed or addressee. A letter does not necessarily have to be short, even though that is what it is called: The Latin word "brevis" means "short".
Even in ancient times, people sent letters to each other. However, this was an expensive thing to do, at least if the recipient lived far away. It was not until the 19th century that the postal service came into being as we know it today.
You put the letter in an envelope and write on it who should receive it. You put stamps on it that you bought beforehand. That way, the post office knows that you paid for it to be sent. In the past, the recipient had to pay. You put the letter in a letterbox or hand it in to the post office staff. The post office staff then make sure that the letter gets to the recipient, where a postman delivers it.
Another important thing about a letter is how it is written and what it says. There are rules so that the recipient knows immediately what he or she has received. First of all, the writer should write who he is and what his address is. Some people have their own letterhead for this purpose, on which this is already pre-printed. Then there is when and where to write. The addressee is addressed with a salutation, such as "Dear Mrs. Müller" or "Dear Felix". The letter text is followed by a greeting and your own signature.
Nowadays, many people write e-mails instead of letters. However, some people prefer to send letters on paper if it is about something very important or special. This can be a love letter, a birthday greeting or a consolation after a bad event. Letters are very important for those who research history. Much is only known about people from earlier times because letters have been preserved.