A fax (facsimile) is like a letter except it is delivered electronically on a fax machine. The main difference in format is the additional piece of information that tells how many pages were sent as part of a fax.
Fax (short for facsimile), sometimes called telecopying, is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer or other output device. The original document is scanned with a fax machine (or a telecopier), which processes the contents (text or images) as a single fixed graphic image, converting it into a bitmap, and then transmitting it through the telephone system. The receiving fax machine reconverts the coded image, printing a paper copy. [wikipedia.com]
Example of Fax:
