Every plugin should be placed to the IDEA_HOME\plugins folder. It may be in jar or just a folder.

Plugin is in jar:

Plugins Sample.jar/ com.intellij..... ... ... META-INF plugin.xml

Plugin is a folder:

Plugins Sample lib classes com.intellij..... ... ... META-INF plugin.xml

Classes and lib folders are automatically added to the classpath.

Plugin configuration (plugin.xml)

<!-- plugin name -->
<!-- description -->
<!-- plugin versuin -->
<!-- minimum and maximum IDEA version plugin is supposed to work with -->
  <idea-version min="3.0" max="3.1"/>
<!-- application components of the plugin -->
<!-- component implementation class -->
<!-- component interface class -->
<!-- project components of the plugin -->