An analog signal is defined for a continuum of values of domain parameter and it can take a continuous range of values.
A digital signal is defined at discrete points and also takes a discrete set of values.
As an example, consider the case of an analog-to-digital (AD) converter. The input to the AD converter is an analog signal while the output is a digital signal obtained by taking the samples of the analog signal at periodic intervals of time and approximating the sampled values by a discrete set of values.
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Figure 3 Analog-to-digital (AD) converter
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