A sadist is a person who is sexually aroused or gains sexual gratification by inflicting pain, cruelty, or humiliation on others. Sadists can also experience the same gratification from watching someone else do these things.
Sadism is believed by some to be a paraphilia or form of sexual disorder in which people gain gratification from objects or situations not normally deemed sexual. Others, however, feel that sadism should not be considered a disorder and can be a safe part of sex play as long as it does not harm or cause distress to either partner.
More About Sadist
In the BDSM (bondage, dominance, masochism, sadomasochism) community, the consensual use of sadomasochistic practices is common. For submissives, allowing someone else to control their sexual pleasure can be a turn on, while some measure of pain is said to heighten sensation, excitement, and therefore, sexual arousal. For the dominant in the situation, being in control of another's person pleasure - and to some extent their bodies and minds - is also a stimulating part of the game. In BDSM, the consent and comfort of both partners tends to be valued, and play often starts with the establishments of limits and safe words to ensure both participants' safety and integrity.