Sexual Sadism is "the tendency to derive pleasure, especially sexual gratification, from inflicting pain, suffering, or humiliation on others".  Sadists can be either exclusive or non-exclusive (the former meaning they require the infliction of pain to gain arousal, the latter meaning that while preferred, it is a completely optional part of their sexual experience). The APA considers this to be a paraphilia, though their diagnostic criteria have changed recently. "Sadistic fantasies or acts may involve activities that indicate the dominance of the person over the victim (e.g., forcing the victim to crawl or keeping the victim in a cage). They may also involve restraint, blindfolding, paddling, spanking, whipping, pinching, beating, burning, electrical shocks, rape, cutting, stabbing, strangulation, torture, mutilation, or killing". 
Types of Sadism
There are basically two different kinds of sadists. The first kind are those who require their victims to greatly enjoy the pain inflicted on them (i.e. those who enjoy masochism) in order to be aroused. This requires consent of the victim. The second kind are the opposite- those who require their victims to hate the pain inflicted on them, and obviously require nonconsenting victims. The second type are the majority of men imprisoned for rape, even though only 5% of rapes end in imprisonment.
Legality of Sadism
Consentual sadism is legal in the United States because the consent of the victim is considered a valid defense in court for all crimes except for Capital crimes. Since murder is a Capital crime, it remains the sole exception in that one cannot legally kill someone who wants to be killed during sex. Canada and the United Kingdom do not have this clause; therefore, any matter of sadism is illegal in those countries.
Morality of Sadism
Sadists who inflict pain on partners who are equally willing and eager to receive it are moral. They give their partner precisely what (s)he wants. Sadists who inflict pain on victims who hate it have a much simpler name: rapist. The immorality of rape is obvious.
The shifting of the APAThe diagnostic criteria in the DSM-IV does not include nonconsent as a requirement, and the description acknowledges the possibility of practicing sadism with a consenting victim. The diagnostic criteria in the DSM-5 however does require nonconsent. The reason the APA offers for this change is that a diagnosis of sadism should only be of the immoral kind, because Child Protective Services have been known to separate families based on such a diagnosis.
- APA. DSM-IV; APA; Washington, D.C. pg. 530, (January 1995)