Currently, the CRAN package repository features 20131 available packages.
CRAN Task Views aim to provide some guidance which packages on CRAN are relevant for tasks related to a certain topic. They provide tools to automatically install all packages from each view. Currently, 44 views are available.
Please type help("INSTALL") or help("install.packages") in R for information on how to install packages from this repository. The manual R Installation and Administration (also contained in the R base sources) explains the process in detail.
The manual Writing R Extensions (also contained in the R base sources) explains how to write new packages and how to contribute them to CRAN.