You don't need a fax machine or phone line to send a fax. The best online fax services allow you to send and receive faxes via your email address, a mobile app, or a web portal—and sometimes for free.

A fax that is sent via a computer running fax software to a fax machine using dial-up modems and analog phone lines. To receive a fax, the computer and fax application must also be running. See ...