A plug-in is used to group your code into a modular, extendable and sharable unit.
Plug-ins are modular as each plug-in contains some portion of code. The plug-in specifies other plug-ins (or java packages) it requires to be available to run and it also specifies the set of java packages it provides. An Eclipse based program or product will contain multiple plug-ins, which can be added, replaced or removed to alter the functionality of the program.
Plug-ins are extendable using extensions and extension points. A plug-in can provide one or more extension points so other plug-ins can add to the functionality of the plug-in. A plug-in may also provide extensions to connect to other plug-ins.
Plug-ins are sharable. A plug-in can be exported as a directory or as a jar which can be added to other applications. Plug-ins can be grouped into features which can be distributed and installed into applications.
Eclipse plug-ins are based on OSGi bundles. OSGi is used to manage the plug-ins in an Eclipse application. A plug-in must contain a manifest file with valid OSGi headers for plug-in name and version. Extensions and extension points functionality added by Eclipse in addition to OSGi. To use extensions you must provide a plugin.xml file. PDE provides a full featured project and editor for creating and editing these files.
New Plug-in Project