A “control voltage”, or “CV” for short, is the name given to an electrical signal used to control some aspect of a synth. As opposed to audio signals, CVs are usually fairly low frequency (slow), sometimes even being a constant, unchanging voltage. Whereas amplitude in an audio signal simply corresponds to loudness, and can be amplified or attenuated without changing the important content of the signal (the audio). By contrast, amplitude is all that matters for a CV. 2V means 2V, and amplifying this signal or attenuating it (including accidental attenuation e.g. from a passive mult) can change the meaning and effect of the signal completely. An example of a particularly precise CV is the V/Oct CV used for tuning. A voltage change of just a few percent can render it useless for accurate tuning.
CVs are often used to control the intensity of a given effect in a module. For example, one oscillator may be patched into another for FM synthesis, and another CV signal may be used to control the intensity of the frequency modulation.
Gate / Trigger
“Gate” and “Trigger” are common types of CVs used to signal an event, for example, a drum hit or the beginning of a note played.
A trigger is for one-shot events where duration is irrelevant, like a drum hit. The event is activated on the rising edge of the trigger input. The duration that the signal is held high has no effect, only the onset of the trigger.
A gate is similar to a trigger, except it controls both the start and the end of the event, e.g. how long a note is held. An envelope generator would use a gate input to keep track of when a key is pressed and released (On the rising and falling edge respectively).