|base||Binaries for base distribution. This is what you want to install R for the first time.|
|contrib||Binaries of contributed CRAN packages (for R >= 3.4.x).|
|old contrib||Binaries of contributed CRAN packages for outdated versions of R (for R < 3.4.x).|
|Rtools||Tools to build R and R packages. This is what you want to build your own packages on Windows, or to build R itself.|
Please do not submit binaries to CRAN. Package developers might want to contact Uwe Ligges directly in case of questions / suggestions related to Windows binaries.
You may also want to read the R FAQ and R for Windows FAQ.
Note: CRAN does some checks on these binaries for viruses, but cannot give guarantees. Use the normal precautions with downloaded executables.