What is the main difference between ARDUINO and PLC?

Both are used to control, they such like of a classic car and a new one 

PLC      : Reliable, faster, high-power.  Used in factories

Arduino: Cheap, easy to program, small/nano. Used in hobbies/  home appliances

PLC is a special microcontroller designed for industrial application. It is for controlling machinery or processes. Arduino is a microprocessor that can be used for any type of application. The basic difference between PLC and Arduino is only the way of programming. Most common way of PLC programming is with graphical language Ladder logic programming which looks a little more similar to electrical schematics than a computer programming language.