This chapter presents an overview of the analog and digital signals. A digital or binary, signal can have only two possible specified levels or states:—an “on” state, in which the signal is at its highest level and an “off” state, in which the signal is at its lowest level. Analog signals contain information within the variation in the magnitude of the signal with respect to time. The relevant information contained in the signal is dependent on whether the magnitude of the analog signal is varying slowly or quickly with respect to time, or if the signal is considered in the time or frequency domains. A digital pulse train is described as a special type of digital signal, comprising a sequence of digital pulses. Like all digital signals, a digital pulse can have only two defined levels or states. It is defined as a pulse because it remains in a nonquiescent state for a short period. This chapter explains in detail, the classification of signals. It also introduces analog signal and analog AC signals. Characteristics of resistive temperature detectors are also discussed in the chapter. Some other topics discussed include sensors and transducers as well as thermocouple types and standards.