To be a proficient R user, you need to read and understand the material in the book Advanced R by Hadley Wickham. The second chapter in this book is on vocabulary - a list of functions from the base, stats and utils packages which all R users should be familiar with.
In this post, we explore the functionality provided by the plyr package. The ideas behind this package are described in this paper by Hadley Wickham. However, rather than trying to understand the theoretical underpinnings of the package, we look at some of the useful functions provided by this package and how they work.