Modem
Modulator-Demodulator
A device that converts between analog and digital signals to connect to the internet.
In Simple Terms
A modem is a device that converts the digital data from your computer into analog data that can travel over phone lines, connecting you to the internet. The data inside your computer or phone is digital, but the signals traveling through outside wires are analog waves. A modem translates between these two different types of signals so they can communicate with each other.
Behind the Name
The name modem comes from combining pieces of two words: modulator, the part that turns digital signals into analog signals, and demodulator, the part that turns them back into digital signals. Put "mo" and "dem" together, and you get modem.
Take a Closer Look!
A modem is a device that converts between the "digital signals" used by computers and other devices and the "analog signals" that travel over phone lines, connecting them to the internet.
Since the data inside your computer and the data flowing through outside phone lines are different in form, this device is needed to bridge the two.
When your computer sends data out, the modem changes the digital signal into an analog wave signal so it can be carried over the phone line.
Conversely, when data arrives from outside, the modem does the reverse: converting that analog signal back into a digital signal your computer can understand.
By transforming outgoing signals and converting incoming signals back to their original form, a modem repeats this process to carry out communication.
Broadly speaking, you could say a modem works like a "translator" between machines that speak different languages.
High-speed fiber-optic lines are widely used for internet connections these days, and in that case a device called an ONU (Optical Network Unit) handles the conversion between optical and electrical signals instead.
Both devices share the same goal of turning your computer's data into a signal suited for the outside line, but an ONU communicates using flashes of light, so it works on a different mechanism than a modem.