A filter is exactly what it says on the tin: a process applied to audio to 'filter' out certain parts of a frequency range or allow others to pass through. A high-pass filter, for example, scoops out the low frequencies and allows the high ones to pass; a low-pass filter does the opposite. A band-pass filter allows a certain bandwidth of frequencies through while a notch filter attenuates or removes a certain range of frequencies.
The main filter control is the cutoff frequency - where the low- or high-pass action takes place. High-pass filters allow all frequencies above the cutoff to pass; low-pass filters allow everything below to pass through.
The other important filter control is resonance. Less easy to understand, this is essentially a boost of the frequencies around the cutoff frequency. Adding resonance at this point increases the emphasis and drama of the sound. Add it as you sweep through on a low-pass filter and it adds a more dramatic swirling effect. Sweeping through cutoff and resonance controls on a filter has become a staple dance music effect - see more below.
The filter is the core ingredient of an EQ plugin and there is some crossover between the plugins we have included above and the best EQ Plugins listed elsewhere. However, the broad difference between filters and EQs is that filters can be more creative, extreme and can add more modulation and character, so can often deliver way more dynamic movement and extreme analogue saturation. You'd typically use an EQ to subtly enrich a track or stereo mix, whereas a filter might be used to deliver the kind of full-on, in your face effects so prominent in club music.
For dance music, for example, on a very basic level, the filter can be used to sweep through a club mix using a low- or band-pass effect to gradually and dynamically raise certain frequencies within the beats, bass, vocals, pads or even the entire mix. Think of a typical build/drop - the filter provides that dramatic build on some or all parts of the mix. Or apply a dramatic filter to a bass sound to create some of the most twisted sounds used in bass-heavy music. As such, many of the filters in our buyer's guide have become firm favourites with dance music producers over the years. Indeed, this also explains the longevity of some of our choices; FilterFreak and Volcano have been with us for the best part of two decades!
Your choice between using a filter and an EQ, then, lies in how creative and extreme you want to get with your frequency processing. Generally, the more creative you want to be, the more you should lean towards buying a filter, although you can get very creative with some EQs as well.