What are inverse kinematics and rigging?

Question 1

Why Forward Kinematics are not used?

Answer 1

it takes long time and key frames to animate.

Question 2

What is the advantage of IK relative to FK?

Answer 2

1) IK has controller (guide layers)
which are animated by Key frames only and IK does the rest of animation.
2) need to deal with 6 guide layers.