This official blog post from the U.S. Department of State has an authoritative discussion. It discusses yesterday's "Implementation Day" as being when Iran was verified to be in compliance with its nuclear commitments. As a result, sanctions were lifted by the international community.
|Secretary Kerry discusses the plan controlling Iran's nuclear program.|
This terrific video (3:33) from the Wall Street Journal in September 2015 explains what was agreed to in the deal. It would make an excellent introduction to your students.