A pn junction diode is a two-terminal semiconductor device that allows current to flow easily in one direction and blocks it in the other. It consists of a pn junction with connecting leads. When forward biased, current flows through the junction. When reverse biased, it acts as an open switch. Diodes come in low, medium, and high current types depending on their size and the amount of current and reverse voltage they can handle. The document discusses the characteristics and parameters of diodes like forward voltage drop, reverse saturation current, and breakdown voltage. It also presents the forward and reverse bias characteristics and equivalent circuit models of diodes.