all' hileth', Hephaiste; didou d'areten te kai olbon.* --Homer, 7th century BCE

The premise of roxygen2 is simple: describe your functions in comments next to where their definitions and roxygen2 will process your source code and comments to produce R compatible Rd files. Here's a simple example from the stringr package:

#' The length of a string (in characters).
#'
#' @param string input character vector
#' @return numeric vector giving number of characters in each element of the 
#'   character vector.  Missing string have missing length.
#' @keywords character
#' @seealso \code{\link{nchar}} which this function wraps
#' @export
#' @examples
#' str_length(letters)
#' str_length(c("i", "like", "programming", NA))
str_length <- function(string) {
  string <- check_string(string)

  nc <- nchar(string, allowNA = TRUE)
  is.na(nc) <- is.na(string)
  nc
}

When you roxygenise your package these comments will be automatically transformed to the Rd file you need to pass R CMD check:

\name{str_length}
\alias{str_length}
\title{The length of a string (in characters).}
\usage{str_length(string)}
\arguments{
  \item{string}{input character vector}
}
\description{
  The length of a string (in characters).
}
\seealso{\code{\link{nchar}} which this function wraps}
\value{numeric vector giving number of characters in each element of the
character vector.  Missing string have missing length.}
\keyword{character}
\examples{
  str_length(letters)
  str_length(c("i", "like", "programming", NA))
}

library(roxygen2)
roxygenize('<package>')

By default, roxygen2 will create files in the package man directory, although if you're worried you can override the defaults to copy the complete package to a new directory and create the files there.

roxygen2 comes with three roclets, three tools for parsing your source code and producing files useful for documenting your package:

• collate_roclet: allows you to add @include directives to ensure that files are loaded in the order they are needed

• namespace_roclet: creates your NAMESPACE automatically. 95% of the time all you need to do is label functions, methods and classes that you want to export with the @export tag

• rd_roclet: produces Rd files by inspecting both function definitions and roxygen2 comments in the source code.

By default, roxygenise will run all three, but you can choose which ones to run using the roclet parameter. It's also possible to write your own roclets - more on this in the future.

• Tobias Verbeke, for catching an unadorned URL.

• Hail, Hephaistos! Grant skill and weal.