This document discusses signals and systems. It defines signals as physical quantities that vary with time and/or space and carry information. Signals are classified as one-dimensional, two-dimensional, or multi-dimensional. They can also be periodic or non-periodic, even or odd, energy-based or power-based, deterministic or random, and causal or non-causal. Standard signals include sinusoidal, exponential, unit ramp, unit impulse, and unit step signals. Discrete time signals are defined at specific time instances while continuous time signals vary continuously with respect to time. Systems are defined as sets of connected components that produce outputs in response to inputs. Applications of signals and systems include communications, controls, signal processing and more.