This document discusses digital filters. It begins by defining digital filters as electronic circuits that perform signal processing functions by removing or enhancing certain frequency components of a sampled, discrete-time signal. It then outlines four basic types of ideal digital filters defined by their magnitude responses. The document also discusses different classifications of digital filters based on characteristics like linearity, time-invariance, and structure. It provides examples of common filter structures like direct form, cascade form, and parallel form. Finally, it briefly compares digital and analog filters, noting advantages and disadvantages of each.