An oscillator is an electronic circuit which produces periodic or regularly repeating waveforms; i.e. sine, square, sawtooth, or triangle waves. An LFO is an oscillator producing these waveforms at a very low frequency or pitch. These slowly vibrating, generally subsonic waves (0 – 20 Hz or so) are often used to modulate or change a parameter in a synth, sampler, or effects processor. One common application is modulating the pitch of an audio oscillator with an LFO; this results in vibrato. If the volume of an audio oscillator is modulated with an LFO, the result is tremolo. Just about any time you see a “modulation” control on a device, it is controlling an LFO, and being used to periodically change some parameter.