Information was already being recorded electronically before computers were introduced. Magnetic tapes where commonly used to record sound. During recording the magnetic strength of a magnetised tape was modified as it passed a recording head. The net result was that the magnetic strength along the tape was made to vary, with same pattern that was present in the sound wave that was being recorded. This direct copying of the pattern contained in original information, is referred to as analogue recording. (Another example of analogue recording, is a conventional film camera, where the pattern in the image is transferred directly onto the photographic film). In digital recording a different method is used, the information is stored as a series of numerical codes, from which the original pattern can subsequently be reconstructed. E.g. in digital audio recording, the sound level is measured repeatedly many times a second and these values are then stored in a digital file. (In a digital camera the image is broken down into tiny picture elements (pixels). The colour and brightness of each pixel is then measured and recorded).
The image above uses colour, to display the smooth variation in the strength and polarity of the magnetic particles along the tape. The four values shown below the waveform represent measurements for a digital recording method. It is clear that in practice, many measurements must be made very quickly, in order to record enough values to accurately "reconstruct" the waveform at a later time.