Question

What is a main difference between a plugin and a component in extjs ?
When should I implement and use some behavior as a class and when as a plugin ?

Answer 1

Well that is more a conceptional question than related to any programming issue. But it can be answered in two sentences:

• A Plugin modifies or extend the behavior of a component at creation time without the need to extend the whole class
• Plugins can be mixed, meaning a component can include many plugins which allow you to instantiate many component instances where each may have different behavior but all uses the same component class

Like @A1rPun mentioned

Plugins allow us to create reusable code that will modify or add to a
component’s look and behavior during or after its instantiation.