Often, you will want to animate only part of a character, such as the arm and not the entire body. You can do this using IK nails, which are IK constraints that temporarily cut or constrain the chain. With the IK nails, you can cut an IK chain or block a part on the spot. For example, you can fix a character's feet to the ground when he walks.

There are different types of constraints you can place on the chain:

IK Nail: Blocks the X, Y and Z positions.
Hold Orientation: Blocks the angle.
Hold X: Blocks only the X position.
Hold Y: Blocks only the Y position.