Digital filters are systems that implement DSP algorithms using hardware and/or software to operate on discrete input signals, usually samples of continuous signals. There are two main classes of digital filters: FIR (Finite Impulse Response) filters and IIR (Infinite Impulse Response) filters. FIR filters are described by difference equations that define the output as a finite weighted sum of previous input samples. IIR filters are described by difference equations that define the output as a weighted sum of previous input and output samples. Digital filters can be implemented using different structures, including direct form, cascade form, and lattice form structures.