Packages are one of the best things in R. They add new functions and features to the language environment and extend its applications over many different use cases and domains. Packages are supported by a large community of developers and allow R to connect to many different external algorithms and libraries—many of them even written in different programming languages.
Contributors all over the world including developers or domain experts in physics, finance, statistics etc. create a lot of additional content, such as custom functions for specific use cases. These functions, together with documentation, help files and datasets can be gathered into packages. Packages can be made public through package repositories so that anyone can install and use them. The most popular package repository is CRAN which hosts over 15,000 packages.