Need help with IK animation

I'm trying to lear Inverse Kinematics animation, I'm searching for a good tutorial to learn, but I didn't find one for starters! Can you explain me basics of IK unity system! I found on ScriptingAPI page this example:

Code (CSharp):

1. using UnityEngine;
2. using System;
3. using System.Collections;
4.  
5. [RequireComponent(typeof(Animator))]
6.  
7. public class IKControl : MonoBehaviour {
8.    
9.     protected Animator animator;
10.    
11.     public bool ikActive = false;
12.     public Transform rightHandObj = null;
13.     public Transform lookObj = null;
14.  
15.     void Start ()
16.     {
17.         animator = GetComponent<Animator>();
18.     }
19.    
20.     //a callback for calculating IK
21.     void OnAnimatorIK()
22.     {
23.         if(animator) {
24.            
25.             //if the IK is active, set the position and rotation directly to the goal.
26.             if(ikActive) {
27.  
28.                 // Set the look __target position__, if one has been assigned
29.                 if(lookObj != null) {
30.                     animator.SetLookAtWeight(1);
31.                     animator.SetLookAtPosition(lookObj.position);
32.                 }  
33.  
34.                 // Set the right hand target position and rotation, if one has been assigned
35.                 if(rightHandObj != null) {
36.                     animator.SetIKPositionWeight(AvatarIKGoal.RightHand,1);
37.                     animator.SetIKRotationWeight(AvatarIKGoal.RightHand,1);
38.                     animator.SetIKPosition(AvatarIKGoal.RightHand,rightHandObj.position);
39.                     animator.SetIKRotation(AvatarIKGoal.RightHand,rightHandObj.rotation);
40.                 }      
41.                
42.             }
43.            
44.             //if the IK is not active, set the position and rotation of the hand and head back to the original position
45.             else {        
46.                 animator.SetIKPositionWeight(AvatarIKGoal.RightHand,0);
47.                 animator.SetIKRotationWeight(AvatarIKGoal.RightHand,0);
48.                 animator.SetLookAtWeight(0);
49.             }
50.         }
51.     }  
52. }
53.  

It worked for ThirdPersonCharacter from Standard Assets but I don't know how to extend it to more complex skeleton because AvatarIKGoal has only 4 proprieties and SetLookAtWeight for head position! But The character has thumbs, palms, shoulders, etc. And I don't know how to animate them!