IK vs FK?

I like to think about it as one of those snake toys.

When you are on Snake FK, you have to move each part independently to make the snake slither:

You have individual control for each section and you have to manually move it all.

When it's Snake IK, you only control the head and the rest of the body will slither on it's own as it should. Less control but you don't have to worry about it all so much.

Haha - make sure you finish this one - you're nearly there.
In the new shiny course, one of the first things I say is to go and watch this one before tackling that one anyway.

Let's see if I can answer those questions.

IK vs FK.
IK stands for inverse kinematics and FK stands for forward kinematics.
What's the difference?  FK flows from the top of the hierarchy down.  So if you move the top bone, all the ones below it follow along.
It's good for arms that don't interact with anything or any part of your character that needs this type of behaviour (like and antenna on top of a robot's head)
IK works in the reverse order. Imagine a foot sticking to the ground.  When you move the torso, you want the foot to stay planted on the ground and all the leg bones to adjust to stay attached between the hips and that foot.  And if you want to pick that foot up and kick a ball, then you don't want that movement to affect the hips, only the legs.  That's what a IK chain is for.

Are you locked in to one or the other?

Well kind of.  If you, the rigger, don't build your rig with an IK setup, then an animator can't control it in a IK way.
(And vice versa)
So what do you do when you need both?  You rig it with both!  Then you're not locked in.
But you only need to do that on parts that makes sense.

I think I do explain the basic mechanism behind that in this course - but if not it is in the next course.  I show on a really simple example, and then later we build a super dooper one for the arms and the legs of Luna.

Hope that helps.