At the most fundamental level, a modem is a device which sends and receives information between two different computers. Today's primary use for modems is to connect computers to the Internet via phone lines. However, modems are used in any capacity of transmitting analog signals, anywhere from light-emitting diodes to radio. Digital modems first arose with the need for transmitting data for North American air defense during the 1950's. Mass production began in 1958, connecting terminals with various air bases, radar sites, and such. In 1963, the first commercial modem was manufactured and was sold by AT&T. For a time, dial-up modems were the most common tool for accessing the internet, however as newer forms of accessing internet have become popularized, the modem itself underwent a change. For more information on the history of the modem, click here Today, they are most often used in conjecture with wireless internet, or WiFi. For a comparison of cable modems and wireless modems, see the two images southeast of here:
Without a modem, one cannot connect to the internet. For example, without a modem, even if your video card and monitor work, one couldn't stream any video content, because they wouldn't actually be connected to the internet.
Computers run on purely digital data, relying on number sequences to transmit information, but phone lines, even today, still contain at least in part analog data, relying on a variety of electrical signals to be carried. In order to bridge the two to create access to the internet, a modem is required. A modem has two plugs in it: one to connect to the phone line (or the cable provided by your ISP), and one to connect to your computer (or WiFi router). This article explains in much more detail how the modem works.
The word "modem" is a portmanteau of the words "modulator" and "demodulator." Usually, modems are used to send digital data over a phone line. Two modems are at play: one that modulates digital data from a computer into electrical signals that are compatible with the phone line. Information carried on electrical signals is added to a carrier wave, and brought to the receiving modem, as displayed on the image southeast from here:
To the right over there is an example of amplitude modulation, in which the carrier wave is modified to support the varying amplitude of the input electrical signal.
The other, receiving, modem demodulates the information back into digital data, which is then carried back to the computer. Information is constantly exchanged between the two over carrier waves in order to provide access to the internet.
Look down and to the right for a really small of the modulation and demodulation process:
Inside its protective case, a modem looks like this. One might say it resembles a motherboard, which you can learn about by clicking here.
Being that modems are a modern necessity, one might consider clicking the link here to learn about which type of modem might be most suitable for their needs.