Short answers, pulled from the story.
Hedonism is a family of philosophical views that prioritize pleasure. It includes psychological hedonism about motivation, axiological hedonism about value, and ethical hedonism about right action, with the word deriving from the Ancient Greek hēdonē.
Psychological hedonism is the theory that all human behavior is motivated by maximizing pleasure and minimizing pain. Axiological hedonism holds that pleasure is the sole source of intrinsic value, and ethical hedonism argues that people have a moral duty to pursue pleasure and avoid pain.
Aristippus of Cyrene, who lived from 435 to 356 BCE, is usually identified as the earliest philosophical proponent of hedonism. The view was also developed by the Cyrenaics and Epicureans in ancient Greece, the Charvaka school in ancient India, and Yang Zhu and Yangism in ancient China.
The paradox of hedonism is the thesis that the direct pursuit of pleasure is counterproductive. It holds that conscious attempts to become happy usually backfire, and the best way to produce pleasure may be to follow other goals and let pleasure arrive as a by-product.
The hedonic treadmill is the theory that people return to a stable baseline of happiness after significant positive or negative changes to their circumstances. Studies on lottery winners suggest their happiness rises with new wealth but returns to its original level after about one year.
Jeremy Bentham developed classical utilitarianism and the hedonic calculus, focusing on the intensity and duration of pleasure for the greatest good of the greatest number. His student John Stuart Mill added quality as a factor, arguing that higher pleasures of the mind are more valuable than lower pleasures of the body.
Outside academic philosophy, hedonism is often used as a pejorative for an egoistic lifestyle seeking short-term gratification, such as indulging in sex and drugs without regard for the consequences. This folk hedonism is associated with a lack of foresight about potential harm, and most philosophical hedonists reject the idea that it leads to long-term happiness.