Prior to digital technology, electronic transmission was limited to analog technology, which conveys data as electronic signals of varying frequency or amplitude that are added to carrier waves of a given frequency. Broadcast and phone transmission has conventionally used analog technology.
Digital technology is primarily used with new physical communications media, such as satellite and fiber optic transmissions. One example of an Analog-to-Digital converter is a modem, which is used to convert the digital information in your computer to analog signals for your phone line; converting analog phone signals to digital information for your computer.
When you record your voice or use a VoIP solution on your computer, you are using an analog-to-digital technology to convert those vocal inputs (analog) into a digital form. An analog signal can be represented as a series of sine waves. The term originated because the modulation of the carrier wave is analogous to the fluctuations of the human voice or other sound that is being transmitted. The term analog, also describes any fluctuating, evolving, or continually changing process. Any signals in the "real world" such as light, sound, etc. can be considered analog.
Digital signals propagate more efficiently than analog signals, largely because digital impulses, which are well-defined and orderly, are easier for electronic circuits to distinguish from noise, which is chaotic. This is the chief advantage of digital modes in communications. Computers "talk" and "think" in terms of binary digital data, creating the need for ATD converters. In other words, analog signals must be turned back into digital form (demodulated) at the receiving end so that the computer may process this data in it's digital format.