Electronic filters are circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. Electronic filters can be:

• passive or active
• analog or digital
• high-pass, low-pass, band-pass, band-stop (band-rejection; notch), or all-pass.
• discrete-time (sampled) or continuous-time
• linear or non-linear
• infinite impulse response (IIR type) or finite impulse response (FIR type)