Beginning with Edison's invention of the phonograph in 1877, a number of technologies have been used to capture and reproduce sound waves, and they fall into two broad categories: analog and digital. To understand the difference between analog and digital, let's think for a minute about a clock, which is probably the most common example of a technology with both analog and digital equivalents.
Imagine an analog wall clock with the three traditional hands, designating hours, minutes, and seconds. The hands move smoothly; as the second hand sweeps around the dial, the minute hand makes its almost imperceptible progress from one-minute mark to the next. The motion of the clock is like time itself, smooth and without interruption.
Digital clocks measure time in precise increments. They convey no sense of the smooth flow of time; the digits representing the seconds change instantly, one after another, and after sixty seconds have passed, the minute digit increments immediately. A digital clock chops up the uninterrupted flow of time into precise units--hours, minutes, seconds, tenths of seconds, and even hundredths or thousandths of seconds.
The contrast between the analog clock and the digital clock has a parallel in analog and digital sound technologies. The motion of sound waves, like the passing of time, is continuous; sound waves, by nature, are analog. Analog equipment, such as Edison's tinfoil-wrapped cylinder, are able to record and reproduce sound as continuous waveforms. Like the motion of the analog clock, the representation of sound on the cylinder is smooth and uninterrupted. Digital audio equipment, on the other hand, divides the continuous sound waves into discrete samples, captured at precise intervals, just as a digital clock divides time into precise intervals. When the sound is played back, the samples are reproduced in sequence at the same precise intervals, creating the illusion of continuous sound waves.