Eclipse plugins vs features vs dropins

What is the difference between these things from an Eclipse installation?

• plugins
• features
• dropins folder

What is the correct use of these folders?

I usually use dropins for sharing plugins amongst multiple Eclipse installations.
See this article for the dropins directory within an Eclipse.

you can use some different structures to place the plug-ins into the Dropins folder – I prefer to separate them by domain:

/dropins/exampleA/plugins/…
/dropins/exampleB/plugins/…
/dropins/exampleC/eclipse/features/…
/dropins/exampleC/eclipse/plugins/…

But I prefer to reference an external dropins folder through the eclipse.ini.

-Dorg.eclipse.equinox.p2.reconciler.dropins.directory=C:/jv/eclipse/mydropins

The plugins/features directories in Eclipse are the default installation directories for plugin.
For more on the differences between features and plugins, see my SO answer:

• you will copy features (declaration of a group of plugins) in the features directory
• you will copy the actual plugins themselves in the plugins directory.

Within a dropin folder (either the one within eclipse or an external one), you will get back the same plugins/feature structure.

The dropins directory should be used to install a plugin manually. The typical process is that you download a plugin and extract that archive to the dropins directory.

The plugins directory on the other hand is reserved for Eclipse's updater. You should not mess with it manually. Eclipse will store all plugins you install using "Help"->"Software Updates" or "Help"->"Install new software" (depending on your Eclipse version).