signals : if for a signal, the quantities are defined only on a discrete set of time
we call it a discrete-time signal.
in the words, a discrete-time real (or complex) signal can
be seen as a function from the set of integers to the set of real or complex numbers.
a continuous-time real or complex signal is any real-valued or complex valued
function which is defined for all time in an interval, most commonly an infinite interval. signals
analog and digital signal
less formally than the theoretical distinctions mentioned above
two main types of signal encountered in practice are analog and digital.
in short, the difference between them is that digital signal are discrete and quantized
while analog signals possess neither property.