what is the difference between Update & FixedUpdate in Unity?

Question

What is the difference between the Update and FixedUpdate methods, and when should these methods be used?

Answer 1

From the forum:

Update runs once per frame. FixedUpdate can run once, zero, or several times per frame, depending on how many physics frames per second are set in the time settings, and how fast/slow the framerate is.

Also refer to the answer given by duck in the same forum for a detailed explanation of the difference between the two.

It's for this reason that FixedUpdate should be used when applying forces, torques, or other physics-related functions - because you know it will be executed exactly in sync with the physics engine itself.

Whereas Update() can vary out of step with the physics engine, either faster or slower, depending on how much of a load the graphics are putting on the rendering engine at any given time, which - if used for physics - would give correspondingly variant physical effects!