In this August 2010 video, Tom Woods provides an overview of what nullification is and how it works.  In his book, Nullification, Tom shows:

• How the states were meant to be checks against federal tyranny—and how a growing roster of governors and state attorneys general are recognizing they need to become that again
• Why the Tenth Amendment of the Constitution reinforces the rights of states to nullify unconstitutional laws
• Why it was left to the states to uphold the simple principle that an unconstitutional law is no law at all
• Why, without nullification, ordinary Americans will continue to suffer the oppression of unjust, unconstitutional law