The idea behind flanger and chorus audio effects consist in mixing original (dry) signal and one or several delayed (wet) signals and manipulating the delay time.
Original (dry) signal, delayed (wet) signal, resulting output signal.
Both effects allow to make the sound wider and more dynamic, move it to the sides and help listeners to hear better different instruments.
Flanger is a time based effect produced by mixing two identical audio signals when one signal is delayed by a small period of time, usually less than 1 milliseconds to 5 milliseconds, not more than 20 ms. This period of delay is gradually changing using low frequency oscillator (LFO), very often differently for left and right speaker. It produces the effect of district comb filter in motion with even notches across the stereo frequency spectrum. Peaks and troughs of the output signal are in a harmonic series.
This strict mode differs flanger from phaser, which creates deeper and not even frequency notches.
LFO controls define how much (LFO width/range) and how fast (LFO speed/rate) the delay modulation occurs.
Feedback control is usually used when flanging and portion of output signal is sent back to the flanger input for re-processing.
Originally the effect was created using two tape machines that play identical audio signal with one delay on one tape that was produced by putting a finger on a flange of that tape to slow this playback down.
The name of the chorus effect comes from choir, where several similar voices with slightly different timbre and tone occurs together.
Chorus effect is produced by mixing several pitch-modulated copies of the same signal with small delays. Pitch is typically modulated by low-frequency oscillation (LFO) and usually differently in left and right speaker.
Comparing to flanging, chorus effect is created using longer delays, from 5 ms to 25 ms and longer. Controls are pretty much the same, but feedback is not used.