R is an integrated development environment or IDE with the following parts:
R base: this is the core of R. Think of it as an engine on a car. No matter what car you drive, it isn't a car until you put the engine on. So R won't run unless you install this bit. You can start using it instantly as it is already loaded with general statistical analysis tools (functions).
R IDE: this is the thing that connects your thought to R. As I mentioned above, you can use R base directly, but the interface is very simple and minimum. But if you use R IDE app, you can work more easily and efficiently. There are two major choices: R Studio or R Commander. Both have their own pros and cons, but in this book, we will use R Studio.
Add on package: this is the extension of R base. Each package has several distinct functions that have specific purposes. But, as you go along using R, you would find two or more different packages that have the same purposes. In this case, you would need to read their guidelines and search for the testimonies and comments from the user.