RStudio is an integrated development environment (IDE) for R. It includes a console, syntax-highlighting editor that supports direct code execution, as well as tools for plotting, history, debugging and workspace management.

R is a programming language where all operations are performed whereas RStudio is just a user-friendly interface for R. If you are familiar with Cpp then you can understand it by assuming RStudio to be like CodeBlocks and R to be like Cpp.