Short answers, pulled from the story.
Microeconomics is a branch of economics that studies the behavior of individuals and firms in deciding how to allocate scarce resources, and the interactions among them. It focuses on individual markets, sectors, or industries rather than the economy as a whole.
Microeconomics focuses on individual firms and individuals and the markets they operate in. Macroeconomics focuses on the overall level of economic activity, addressing growth, inflation, and unemployment, along with national policies related to these issues.
The distinction was likely introduced in 1933 by the Norwegian economist Ragnar Frisch, who distinguished between "micro-dynamic" and "macro-dynamic" analysis. Frisch was the co-recipient of the first Nobel Memorial Prize in Economic Sciences in 1969.
The first known use of the term "microeconomics" in a published article was by Pieter de Wolff in 1941, who broadened the term "micro-dynamics" into "microeconomics".
The main market structures include perfect competition, monopolistic competition, monopoly, and oligopoly. Other structures include monopsony, a market with one buyer and many sellers, bilateral monopoly, and oligopsony, a market with a few buyers and many sellers.
Opportunity cost is the value of the next-best alternative one could have pursued instead of a chosen activity. It depends only on the value of that next-best alternative, regardless of whether one faces five alternatives or 5,000.
Supply and demand is a model of price determination in a perfectly competitive market, where the unit price settles at the level where quantity demanded equals quantity supplied. Below the equilibrium price a shortage bids the price up, and above it a surplus pushes the price down.