Shiny makes it incredibly easy to build interactive web applications with R. Automatic "reactive" binding between inputs and outputs and extensive prebuilt widgets make it possible to build beautiful, responsive, and powerful applications with minimal effort.
The Shiny tutorial at http://shiny.rstudio.com/tutorial/ explains the framework in depth, walks you through building a simple application, and includes extensive annotated examples.
shiny-options for documentation about global options.