Automatic generation of reports by mixing word processing markup (like
  latex) and S code.  The S code gets replaced by its output (text or
  graphs) in the final markup file.  This allows a report to be re-generated
  if the input data change and documents the code to reproduce the
  analysis in the same file that also produces the report.
Sweave combines the documentation and code chunks together (or
  their output) into a single document.  Stangle extracts only
  the code from the Sweave file creating an S source file that can be
  run using source.  (Code inside \Sexpr{}
  statements is ignored by Stangle.)
Stangle is just a wrapper to Sweave specifying a
  different default driver.  Alternative drivers can be used: the CRAN
  package cacheSweave and the Bioconductor package weaver
  both provide drivers based on the default driver
  RweaveLatex which incorporate ideas of caching
  the results of computations on code chunks.
Environment variable SWEAVE_OPTIONS can be used to override the
  initial options set by the driver: it should be a comma-separated set
  of key=value items, as would be used in a \SweaveOpts
  statement in a document.
Non-ASCII source files must contain a line of the form
\usepackage[foo]{inputenc}(where foo is typically latin1, latin2, utf8 or
  cp1252 or cp1250) or they will give an error.
  Re-encoding can be turned off completely with argument encoding
    = "bytes".