Sadism is a condition in which people get pleasure from humiliating, harming or hurting others. Often the pleasure is of a sexual kind. When a person finds pleasure in being hurt or humiliated, their condition is called Masochism.
The terms "sadism" and "masochism" were both invented by Richard von Krafft-Ebing, a German psychiatrist. He named Sadism after the Marquis de Sade, a French nobleman who in the eighteenth century wrote many books, stories, and plays. Sade's books include many characters who find pleasure in cruelty, including torture and even murder.
These kind of people usually enjoy it when others are hurt, but specific ones love pain in general. Most of the time people who are masochistic or sadistic are sad in real life.
The condition is often fetishized on the internet and sometimes related to self-doubt or self-hate.