This document discusses different types of thyristors including silicon-controlled rectifiers (SCRs), triacs, and diacs. SCRs allow current to flow in one direction when triggered by a positive gate signal and can be used to control both AC and DC circuits. Triacs are bidirectional and can control current flowing in either direction when triggered by a positive or negative gate signal. Diacs are required for triggering triacs due to their nonsymmetrical triggering characteristics. The document provides schematics and diagrams of the different thyristor components.