What are analog signals and digital signals?

(N/A) In electronic circuits such as amplifiers and oscillators,signals exist in the form of continuous changes in current or voltage over time. Such signals are called continuous or analog signals.
$A$ typical analog signal is shown in figures $(a)$ and $(b)$.
If a current or voltage signal has only two discrete values (minimum and maximum),the signal is called a digital signal.
The signal in figure $(c)$ is in the form of a waveform or pulse,which has only two signal levels.
$A$ binary system for displaying such signals is convenient. Binary uses only two digits,$0$ and $1$. The maximum value (e.g.,$5 \ V$) of voltage is expressed as $1$,and the minimum value (e.g.,$0 \ V$) is expressed as $0$.
The input and output voltages indicated by these two values ($0$ and $1$) in a digital circuit are valid.