What is the difference between a transistor,theorist and diode?

its semiconductor devices but different in shape transistor3 layer thyristor4 layer diode2 layer  -pn junction- 

main function for diode is to pass the current or not

thyristor  same as diode but it need ignition switch, it used for higher voltage 

transistor it used for amplification or  as switch in digital circuit  mainly.

Dear Olu Azumi,

After greeting, I was invited here by Said Melouki. You meant thyristor not theorist !!.

You can't compare between diode, Transistor and thryistor as each same technology semiconductor.Diode: is a p-n junction used to direct current in one direction, due to  doping process. used in rectifier application.

Transistor: same technology here but p-n-p/n-p-n  by mean of p-n junction with N-type/P-type  resulting many types MOSFET, JFET and most common type BJT which is used in switch and amplification( i.e. voltage, current gain).

Thyristor: Leads to four layers p-n-p-n all this done to maximize the efficiency by mean like in switching case but as far as I know of Thyristor's applications only involves HVAC and breaker for current overflow.

DIODE" is a1N device with PN junction. It acts like a valve, and allows current flow in only one direction, but not the reverse. It is used to convert a.c. into DC current using the Bridge rectifier circuit. TRANSISTOR is a2N device with NPN and PNP junctions. The central one is called the base in a bipolar or curent biased transistor and is called the Gate in a field effect transistor. A small current or potential on the base or Gate causes a large current across the transistor. Thus the transistor acts like a switch to on and off current and also to regulate the potential. THYRISTOR is a3N device with PNPN junctions. It is used to switch a.c. current at volts and more than4 amperes, thus being a high voltage high power device.