what is the difference between Update & FixedUpdate in Unity?

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

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!

FixedUpdate is used for being in-step with the physics engine, so anything that needs to be applied to a rigidbody should happen in FixedUpdate. Update, on the other hand, works independantly of the physics engine. This can be benificial if a user's framerate were to drop but you need a certain calculation to keep executing, like if you were updating a chat or voip client, you would want regular old update.

Unity has a nice video about Update vs. FixedUpdate here: Unity - Update vs. FixedUpdate

The site is a great resource for beginner game programmers.

Adding to the other answers, Update is in time with the frame rate, not the physics calculation rate. This means that usually when something changes visuals, you want to have it done in Update, not FixedUpdate. This ensures it's always in time for the very moment at which the frame is rendered. I've encountered a number of cases where I had movement that didn't look smooth because I had it calculated in FixedUpdate.

The only potential cost to having something done in Update that should be done in FixedUpdate is that your physics fidelity could drop, especially if you're hitting low frame rates (but in general that's a separate problem that should be fixed, so it's usually not a good reason to put something in FixedUpdate instead of Update).

If you're looking to control the order in which different scripts do their Updates, FixedUpdate is the wrong tree to bark up; you should be looking at something like EarlyUpdate instead.

Update(): contains that code which has to be executed with visual rendering

FixedUpdate(): contains that code which has to be in sync with physics interaction

source : https://youtu.be/noXtT_zN-84?t=3057