Sublime Plugins are just python files in a folder (named as you deemed appropiate) in the Package folder. A plugin can changed almost everything Sublime Text does, and/or add new functionality.
NOTE THAT: a plugin is written in a pre-installed version of python found in the sublime text folder, this version cannot be upgraded/downgraded by the user. If you want to know which version of python Sublime Text is using when creating a plugin you can type
import syssys.versionIn the console