What is a signal and what are the types of signals?

A signal is a function that conveys information about a phenomenon. Classification: Continuous-time (CT) vs Discrete-time (DT) based on time variable, Analog vs Digital based on amplitude values, Deterministic vs Random based on predictability, and Periodic vs Aperiodic based on repetition. Examples: speech is CT analog, digital audio is DT digital, and power line frequency is periodic deterministic. Signal processing analyzes, modifies, or synthesizes signals for communication, control, and analysis applications.