Question

What do you mean by Revenue?

Hint:

Think "P$\times$Q": TR drives the rest; AR is price; MR is the slope of TR.

Answer 1

Measures: Total Revenue (TR) $= P \times Q$. Average Revenue (AR) $= TR/Q$; in single-price markets, AR equals price. Marginal Revenue (MR) $= \Delta TR/\Delta Q$, the extra revenue from an additional unit.
Market structure link: Under perfect competition, $P$ is constant, so AR $=$ MR $=$ P and the TR curve is a straight line. Under monopoly/monopolistic competition, price must fall to sell more, so MR lies below AR and both slope downward.
Decision use: Profit-maximizing output occurs where $MR = MC$, not where TR is highest.