A traditional fax machine uses a phone line to transmit document data. As more people abandon land lines in favor of cell phones, there is an increasing demand for fax services that don't require land lines.
You can send a fax over the Internet without a land line. There are a variety of fax services that allow you to send faxes from your e-mail by typing the fax number into the address line. Other Internet services let you send faxes from their website by uploading the document and typing in the receiving fax number.
You can also receive faxes without a land line with Internet services. The services will send incoming faxes straight to your e-mail inbox. The fax is usually sent as an attachment.
Internet fax services work on a subscription basis. You pay either by the month, or by batch of faxes sent or received. Some services just charge by the page.