Discrete-time signals are sequences of numbers that represent a signal at distinct intervals of time. Unlike continuous signals, which are defined at every moment, discrete-time signals are sampled at specific points, making them easier to process with digital systems. These signals are commonly used in digital communication and signal processing. In discrete-time systems, each sample can be manipulated using various techniques, such as filtering or modulation. The Nyquist-Shannon sampling theorem is crucial in this context, as it defines the minimum sampling rate needed to accurately reconstruct the original continuous signal from its discrete samples.