What is Eclipse Plugin?

Plugins are the smallest deployable and installable software components of Eclipse.

Each plugin can define extension-points which define possibilities for functionality contributions ( code and non-code ) by other plugins. Non-code functionality contributions are for example the provision of help content.

The basis for this architecture is the runtime environment Equinox of Eclipse which is the reference implementation of OSGI. See OSGi development - Tutorial for details.
The Plugin concept of Eclipse is the same as the bundle concept of OSGI. Generally speaking a OSGI bundle equals a Plugin and vice-versa.

Here are some of the screenshots:

http://www.vogella.com/tutorials/EclipsePlugin/article.html