A diode is the simplest possible semiconductor device. It allows current to flow in one direction but not in the other. Diodes are also commonly known as rectifiers.
There are three common types of diodes:
General Purpose - General purpose diodes permit current to move in one direction. Current to Cathode is forward biased (junction field becomes smaller and current flows). Current to Anode is reversed biased (junction field becomes bigger and current flow ceases).
Zener - Zener Diodes are employed so reverse biased breakdown voltage can be controlled to a limit. The biasing allows zeners to apply current to the anode (junction field will increase to a limit permitting voltage leakage to the set limit). When current is applied to the cathode, a zener diode has the same characteristic behavior of a general purpose diode. They are commonly found as voltage regulators, voltage dividers and for voltage limiting circuits.
Silicon Controlled Rectifier (SCR) - SCRs have a gate to initiate flow but operate similar to general purpose diodes. The current will not flow in either direction unless the control voltage is applied to the gate. When flow begins (cathode to anode), it will remain even after control voltage at the gate ceases. SCR's have 3 leads: cathode, anode and gate.