An effect processor takes an input signal and transforms it into a processed signal. Typically, what you get at the other end (of the effect processor) is a mix of the original unaltered (DRY) signal and the processed modified (WET) signal. When looking at an effect processor control panel, you will always see a DRY/WET knob. If turned all the way to the left (0%), only the input signal comes out. If turned all the way to the right (100%), only the processed signal comes out. Anything in between is a mix of the two signals.
A delay line, as its name applies, takes an input signal and generates a processed signal that's delayed by a certain number of milliseconds, the delay time. A delay effect is not the same as reverberation. Indeed, reverb produces a kind of blurry echo that gets washed out, while a delay line produces a clear echo, that's a priori as clear as the input signal.
Here's what you see when you go into the FX control panel and switch on the DELAY effect.
The SYNC slider switch enables you (when on) to sync the delay time with the beat-per-minute main clock. It means that the delay time will correspond to 1/16 notes, 1/8 notes, 1/4 notes, etc.
The L/R RATIO knob enables you to adjust the left and right delay times.
The FEEDBACK knob controls the amount of feedback. When the processed signal is "fed back" into the input of the effect processor, it is called feedback. With feedback, sounds get repeated forever but the repeating sounds get quieter and quieter (gain less than 1).
Flanger and Chorus
Those two effects are actually similar. They are both delay lines with a modulated delay time. Modulated by what you may ask. By a Low Frequency Oscillator (LFO), of course. When the delay time is modulated, interesting things happen, but let's delve a bit into the effects of adding a delayed signal, for now.
When a delayed signal is superimposed onto a source signal, the frequencies (overtones) are kinda filtered by the effect processor, in other words, the overtones you hear depend on the delay time. A flanger or chorus filters the frequencies in the manner of a "comb filter": some frequencies (peaks) go through amplified while others are attenuated and even canceled (notches). When the delay time is large, the pitch tends to go down. When the delay time is small, the pitch tends to go up. So, when you modulate the delay time, you create pitch modulation. The "depth" on a flanger or chorus processor controls how pronounced the effect is, in essence, the depth of the notches (see the DEPTH knob on both flanger and chorus control panels).
What's the difference between flanger and chorus then?
The flanger has a very small delay time range and produces a "whooshing" sound. The chorus doesn't make the sound sweep of a flanger and produces a "thicker" or richer sound, result of very slightly detuned and out-of-sync voices.
This is the control panel for the FLANGER effect.
The LFO FREQ knob is for adjusting the frequency of the Low Frequency Oscillator (LFO).
This is the control panel for the CHORUS effect.
The chorus effect has the same controls as the flanger, without the feedback (used on a flanger but not on a chorus).