This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A feature is a collection of pluggins. So, you will have both kind of things, usually.
In features you will have all standard installed features, and pluggins the corespondent pluggins.
The dropins folder can contain both, features and plugins. The dropin is usually for hot deployments, if you want to isolate the features and plugins of a product by the standard ones.
Depending on you situation, you can have features and plugins you are interested in either in standard folders or in dropins.