IK Hierarchy Chains

Before we go any further, a word about IK chains. There are three types of chains:

Single Chains
Direct Chains
All Chains

Single Chains

These are the most basic of chains. A single chain is a straight line with no secondary chain attached.

Direct Chains

These link directly to the core of the character rigging. A direct chain is a single chain which goes directly to the core.

All Chains

All chains are attached and move simultaneously when one part is moved.

Related Topics 

Where to Start with Inverse Kinematics?
Inverse Kinematics Tool Properties
Setting up the Character to use IK
Nails
Minimum and Maximum Angle
IK Keyframes
When to use the Inverse Kinematics Tool