What are the differences among triac couplers, SSR (solid state relays), and photorelays?

Triac couplers are a kind of photocoupler used as a trigger mainly for the main triac. SSR (solid state relays) is a semiconductor relay consisting of a triac coupler, main triac, resistance, and capacitor. Triac couplers alone can turn a load ON/OFF. However, the current they can control is so small (around 100mA) that SSR is used for applications for performing ON/OFF control on large currents. Owing to their structure, triac couplers have a tendency to hold a current flowing through the triac once they are turned ON. Even if LED signals are turned OFF after that, the triac couplers will not turn OFF unless the current flowing through the triac becomes a certain current (holding current) level or smaller. For that reason, triac couplers cannot perform OFF control in conjunction with LED signals. Photorelays are also a kind of semiconductor relay. They have a tendency to be able to control both ON and OFF operations in conjunction with LED signals.