What is the difference between a transistor and scr?

SCR or Silicon Controlled Rectifiers are Bipolar junctions Transistor ( Thyristor Family), SCRs are made of two transistors thats why they PNPN or NPNP devices.

The three important parts of transistor are - Emitter, Collector & Base.

& the Three Important parts of SCR are - Anode, Cathode & Gate.

Use of SCRs- rectification of high power AC & power control applications, such as lamp dimming, regulators and motor control.

Use of Transistors - switch, audio, high voltage, amplifiers, switcher