What is a modem? A modem is a small box connected to a computer, or installed inside it, that converts the digital signals from your computer into a form to be sent down the telephone line. The faster the modem, the less time you will spend on the phone sending data like e-mail. Modem speeds are measured in bits per second (bps) and can vary from 14,000bps to 56,000bps. The faster modems are usually more expensive.