This document discusses different types of filters used in signal processing and electronics. It describes filters as circuits that can pass or amplify certain frequencies while attenuating others. There are four main types of filters: low-pass, high-pass, band-pass, and notch filters. Filters are used in applications like radio communications, DC power supplies, and audio electronics. Discrete time filters are described by difference equations and can be either recursive or non-recursive. Recursive discrete time filters are characterized by impulse responses of infinite length and are described using first order difference equations.