Short answers, pulled from the story.
Supply and demand is an economic model of price determination in a market. It postulates that, holding all else equal, the unit price of a good in a perfectly competitive market will vary until it settles at the market-clearing price, where quantity demanded equals quantity supplied. The concept forms the theoretical basis of modern economics.
Augustin Cournot first drew demand curves in his 1838 work Recherches sur les Principes Mathématiques de la Théorie des Richesses. Fleeming Jenkin added supply curves in his 1870 essay on the graphical representation of supply and demand, publishing the first drawing of both curves in English. Alfred Marshall popularized both in his Principles of Economics of 1890.
The Scottish writer James Denham-Steuart first used the phrase "supply and demand" in 1767, in his Inquiry into the Principles of Political Economy. English economics writers avoided the phrase until after the end of the 17th century, and Adam Smith later used it in his 1776 book The Wealth of Nations.
Giffen goods and Veblen goods are exceptions where the demand curve slopes upward. Veblen goods become more attractive at higher prices because of fashion or signalling. Giffen goods are inferior goods that absorb a large part of income, such as potatoes in Ireland, where a rising price has a strong income effect that increases quantity demanded.
The Sonnenschein-Mantel-Debreu theorem proves that when individual demand preferences are aggregated across an economy, the resulting market demand curve can take virtually any shape. It means aggregate demand does not inherit the downward-sloping property of its parts, so equilibrium may not be unique or stable and the law of demand may fail at the macro level.
In the labor market the usual roles reverse: individuals supply their labor for the highest price while businesses demand labor at the lowest, and the equilibrium price is the wage rate. Economist Steve Fleetwood concluded the empirical evidence for these curves is at best inconclusive, citing Kaufman and Hotchkiss in 2006 that for adult men nearly all studies find the labour supply curve negatively sloped or backward bending.