Short answers, pulled from the story.
The four humors in humorism are blood, yellow bile, black bile, and phlegm. Each was associated with a temperament: blood with sanguine, yellow bile with choleric, black bile with melancholic, and phlegm with phlegmatic. Hippocrates described them in On the Nature of Man, and Galen later systematized the theory.
Hippocrates (460-370 BC) is credited with applying the concept of humors specifically to medicine. Galen (129-201 AD) extended and systematized the theory into its most influential form. The Persian polymath Avicenna also made a major contribution through The Canon of Medicine, published in 1025.
Humorism was not definitively disproven until 1858. It began to fall out of favor in the 17th century as advances in cellular pathology and chemistry accumulated. Andreas Vesalius first seriously challenged the theory in 1543, though his criticisms focused primarily on Galen's anatomy rather than the chemical basis of the humoral system.
Humorism linked personality to whichever fluid predominated in a person's body. Excess blood produced a sanguine temperament (enthusiastic, active, social), excess yellow bile produced a choleric nature (ambitious, aggressive, short-tempered), excess black bile produced melancholy, and excess phlegm produced a phlegmatic, reserved character.
Treatment aimed to expel whichever humor was in excess. Methods included bloodletting, purging, catharsis, and diuresis. Galen declared blood the most prevalent humor, making bloodletting especially prominent; the volume extracted ranged from a few drops to several litres over several days. Herbs and minerals were also used, such as chamomile to reduce excess bile.
The four humors shaped character types in European drama from the plays of Menander and Plautus through Ben Jonson's humor plays in the 17th century. Shakespeare's The Taming of the Shrew depicts characters using humoral therapy, and the humors appeared as a recurring theme in paintings, tapestries, and prints. The theory of the four humors also features in Rupert Thomson's 2005 novel Divided Kingdom.