A low frequency oscillator (LFO) produces cyclical waveforms at low enough frequencies to be useful as control voltages. The only way an LFO differs from other classes of VCO is the frequency ranges available - i.e. not all cyclic oscillations lie within the audio frequency range (though this doesn't mean you can't hear them necessarily - a Clock oscillator can sound like a series of regular clicks).
Usually some or all of the following are available - sine, triangle, square, sawtooth waves and random/pseudo-random. An LFO can be used as a modulation source for pretty much any module with a cv input - for instance modulating the frequency of a VCO (vibrato) or amplitude modulation with a VCA (tremolo). It is worth noting many can act as audio frequency oscillators and may have reset, sync and other functions more commonly found on other VCOs.
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