Rtools provides a toolchain for Windows platform that work well with R. It mainly includes GNU make, GNU gcc, and other utilities commonly used on UNIX-ish platform. ow on Linux (and presumably on most other Unices), R downloads the package source and builds the ready-to-use package that is installed on your system right there on your machine. Note that this is a non-trivial process, involving making different kinds of documentation, checking R code for sanity, possibly compiling C/C++ or Fortran code and so on. This in its turn requires a whole series of external tools like make, sed, tar,gzip, a C/C++ compiler etc. collectively known as Rtools.