Straight from the horse’s mouth. Chief Justice John Robert’s Majority Opinion includes this [emphasis added]:
The individual mandate cannot be upheld as an exercise of Congress’s power under the Commerce Clause. That Clause authorizes Congress to regulate interstate commerce, not to order individuals to engage in it. In this case, however, it is reasonable to construe what Congress has done as increasing taxes on those who have a certain amount of income, but choose to go without health insurance. Such legislation is within Congress’s power to tax.
The whole opinion, and the two dissents, can be read here.