How did George W. Bush make torture legal and what did Obama undo?

In the George W. Bush era, how did government organizations get away with using torture? Obviously it was illegal. What legislation did they pass, and what did Obama undo? In this article, it talks about how Obama refused to prosecute those who made torture legal. I guess this implies that it's as simple as changing a law to make torture standard practice?

...Obama refused to prosecute those responsible for sanctioning
torture...