There are internal and external plugins available. External or internal, plugins are specially designed pieces of PHP code which can modify contents of the web page. External plugins are located in folder "_plugins". External plugins must be loaded in "settings.cfg". Internal plugins are loaded automatically.
Some of the loaded plugins do not have to be referenced on your web site. for example external plugin FACEBOOK just makes all your pages more Facebook-friendly for sharing.
But most of the plugins can be references from templates, articles and modules using the following syntax: [(plugin_name) (attributes)](contents)[/(plugin_name)]. We'll have separate topics on each available internal and external plugin.