What is cruel and unusual punishment?

"The Eighth Amendment of the U.S. Constitution's Bill of Rights prohibits the use of 'cruel and unusual punishment.' That phrase was written at a time when burning and beheading were still acceptable legal punishments in some places. Much has changed in America in two centuries. This easily accessible guide examines the context that inspired the Founding Fathers to include this phrase, as well as the concept of 'evolving standards of decency'"--Provided by publisher.