It has one more PN junction. It stays on once it's gate is triggered. The voltage across it's Anode and Cathode must drop to near zero for it to turn off. A transistor will turn off when the base current is removed.
Good uses:
Latching DC switch.
Control of Pulsating DC.
Use as a controllable diode in a rectifier circuit such as those older car battery chargers.
Transistor is simplified as two diodes, NPN when connected by Anodes and PNP when connected by Cathodes.
thyristor is a simple diode with a gate. the diode will do its job whenever the gate triggered. it will stop only when its A-C goes below holding current.
Hi
I think Power is the main factor.For high power Thyristors should be preferred and for low power Transistors(although they being available in high power flavors) but thyristors are just more simpler(including the AC part as indicated by Hero).