Modem

A modem (a portmanteau constructed from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Experiments have even been performed in the use of modems over the medium of two cans connected by a string.
The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone System (POTS), and once received on the other side, converts those sounds back into 1s and 0s. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps".
Far more exotic modems are used by internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optic fibers. Most intercontinental data links now use optic modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second.

Broadband

ADSL modems, a more recent development, are not limited to the telephone's "voiceband" audio frequencies. Some ADSL modems use coded orthogonal frequency division modulation.
Cable modems use a range of frequencies originally intended to carry RF television channels. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel. Typically, 'up' and 'down' signals are kept separate using frequency division multiplexing.
New types of broadband modems are beginning to appear, such as doubleway satellite and powerline modems.
Broadband modems should still be classed as modems, since they do utilise analog/digital conversion. They are more advanced devices than traditional telephone modems as they are capable of modulating/demodulating hundreds of channels simultaneously.
Many broadband "modems" include the functions of a router and other features such as DHCP, NAT and firewall features.
When broadband technology was introduced, networking and routers were not very familiar to most people. However, many people knew what a modem was as most internet access was through dialup. Due to this familiarity, companies started selling broadband adapters using the familiar term "modem".

Voice Modem

Voice modems are regular modems that are capable of playing audio over the telephone line. They are used for telephony applications.

Popularity

A CEA study in 2006 found that dial-up Internet access is on a notable decline in the U.S. In 2000, dial-up Internet connections accounted for 74% of all U.S. residential Internet connections. This figure dropped to 60% by 2003, and currently stands at 36%. Modems were once the most popular means of Internet access in the U.S., but with the advent of new ways of accessing the internet, the traditional 56K modem is losing popularity.