Quote from JamesL:
The Arizona voter referrendum had 2 parts to it: voter ID for in-person voting and a requirement that anyone registering to vote provide proof of citizenship.
SCOTUS struck down the part that violated the citizenship challenge of the law that violated existing federal citzenship requirements for those registering to vote using the "Motor Voter" federal form.
The voter ID part of the Arizona law is still in effect and was not part of this decision.