Your question isn't specific to the law, though I'm sure that's what you intended. You've gotten some very thorough answers for that, so I'd just like to reflect on the more general idea of perspective when it comes to this concept. A kindergartner is likely to see homework as being cruel and unusual punishment, as is a teenager who has to do daily chores around the house. It's all about perspective, and we are fortunate (I'd even say blessed) to have a system whereby reasonable people can present their cases before a reasonable Court which is sworn to uphold the Constitution--the foundational law of our land.
The Eighth Amendment to the Constitution bars the federal government from imposing cruel and unusual punishment for federal crimes and the due process clause of the Fourteenth Amendment keeps the states from doing the same for state crimes. The Supreme Court has the final say in what constitutes cruel and unusual punishment, and in doing so, it usually looks at two aspects of punishment, the amount of punishment and the method of punishment. The Court has said that Eighth Amendment must "draw its meaning from the evolving standards of decency that mark the progress of a maturing society."
As to the method of punishment, the Court usually agrees that excessive physical force constitutes cruel and unusual punishment even if there is no serious injury (Hudson v. McMillian). The Court has said that a person doesn’t even have to suffer physical injury for a punishment to be considered cruel and unusual (Trop v. Dulles). The Court has not said that the death penalty itself is cruel and unusual, and has never declared an execution method to be cruel and unusual, but it has limited the people who may be given the death penalty. The insane (Ford v. Wainwright) and the mentally retarded (Atkins v. Virginia) are two examples of those who cannot be given the death penalty. Mandatory death sentences were found to be unconstitutional because they do not take into consideration circumstances mitigating a death sentence (Lockett v. Ohio)
As to the amount of punishment, the Court has said punishment that is clearly out of proportion to the crime is cruel and unusual. The Court usually applies this proportionality test in cruel and unusual punishment cases. An example of this is the case where a defendant passed a bad check for $100 and received a life sentence without parole due to six previous felony convictions. The Court said the punishment was out of proportion to the crime (Solem v. Helm).
Overall, cruel and unusual punishment has never been defined and is definitely subjective. Some punishments such as dismemberment, breaking bones, and maiming are prohibited while solitary confinement, enforced silence and bad food are allowed.
The courts have pretty consistently defined cruel as the intentional infliction of pain, suffering or humiliation on a prisoner as part of a sentence. The British back in the days of the Empire and pre-independence, pre-Constitution, had very creative and painful ways of carrying out punishments. So if we still tarred and feathered people, that would be both painful and humiliating, a violation of the 8th amendment. The courts have also ruled that prisons have to have reasonable heating and cooling systems, reasonably healthy food, and prisoners have access to exercise and health care.
"Unusual" on the other hand refers more to whether or not is has been commonly accepted practice in the justice system. Some have argued that the electric chair is unusual (I mean, really, who came up with that off the wall idea?), but the courts have said no because they have been used for decades, and since death is instantaneous, do not represent cruelty either. The same "unusual" definition would apply if a judge sentenced a man to sing the Star Spangled Banner during his entire waking hours, or to hop on one leg from 8 to 9 AM each day. Besides being bizarre, it's also against commonly established practice.
The previous thoughts were strong. I would only add that the recent admission of the acceptance of "enhanced interrogation techniques" in Guantanamo Bay and other areas with detainees has broadened the discussion of the 8th Amendment and the phrase "cruel and unusual punishment." The belief that used to lie in the 8th Amendment is that it forbids torture and any type of activity associated with it. Yet, with the construction of the idea that "enhanced interrogation techniques" do not necessarily have to be considered torture, the wide discussion about what exactly "entails" cruel and unusual punishment. Justice Scalia has argued that these techniques are not in violation of the standard in the 8th Amendment because they are not, in of themselves, forms of punishment. This very delicate parsing of words is the reason why the discussion of the 8th Amendment and, in particular, the idea of "cruel and unusual punishment," has taken on new forms and new life which allows for different interpretations of the phrase.
The Eighth Amendment of the U.S. Constitution prohibits cruel and unusual punishment. The interpretation of what is cruel and unusual is left up to the court's. This is always a controversial topic because what one court decides in regards to this can be quite different from the decision of a different court.
This issue often arises in capital cases and over the years many courts have ruled that the death penalty does not equate to cruel and unusual punishment. It also commonly arises from complaints by inmates in relation to jail overcrowding. This to has been addressed and for the most part court's have ruled that prison overcrowding does not amount to a breech of eighth amendment rights.