AUC
AUC is the total amount of drug exposure the body experiences over time, captured as the area under the concentration-versus-time curve. If you sketch the rise and fall of a drug's blood concentration after a dose and shade in everything beneath the line, that shaded area is the AUC. A higher curve, or one that stays elevated longer, encloses more area and means greater exposure.
Crucially, AUC integrates both how high and how long. Two drugs might reach the same peak, but the one that lingers longer has a larger AUC and exposes the body to more total drug. This makes AUC the preferred measure of overall exposure, more informative than the peak alone for many effects and toxicities that depend on cumulative dose.
AUC ties the dose to the body's handling of it: for a given dose, AUC equals the bioavailable amount divided by clearance. Because of this, comparing AUC after an oral dose with AUC after intravenous dosing is the standard way to measure bioavailability, and matching AUC (and peak) is the core requirement for declaring two products bioequivalent.
Bioavailability is calculated by dividing the AUC after an oral dose by the AUC after an equal intravenous dose, since the intravenous route delivers the whole dose to the blood and serves as the 100 percent reference.
AUC ratios convert raw exposures into a bioavailability figure.
AUC measures total exposure (the whole area), while Cmax measures only the highest point; the same AUC can hide very different peak shapes, so both are usually reported.