Analog

Analog describes a device or system that represents changing values. A typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a small number of times (every tenth of a second, for example). In general, humans experience the world analogically. Vision, for example, is an analog experience because we perceive a large number of shapes and colors.

When used in reference to data storage and transmission, analog format is information that is transmitted by modulating a continuous transmission signal, such as amplifying a signal's strength or varying its frequency to add or take away data. For example, telephones take sound vibrations and turn them into electrical vibrations of the same shape before they are transmitted over traditional telephone lines. Radio wave transmissions work in the same way.

Computers, which handle data in digital form,require modems to turn signals from digital to analog before transmitting those signals over communication lines such as telephone lines  that carry only analog signals. The signals are turned back into digital form (demodulated) at the receiving end so that the computer can process the data in its digital format.