What is a plug-in?
In computing, a plug-in is a set of software components that adds specific capabilities to a larger software application. If supported, plug-ins enable customizing the functionality of an application. For example, plug-ins are commonly used in web browsers to play video, scan for viruses, and display new file types. (source: Wikipedia)
What is a component?
A component in the Unified Modeling Language “represents a modular part of a system, that encapsulates its content and whose manifestation is replaceable within its environment. A component defines its behavior in terms of provided and required interfaces” (source: Wikipedia)