write a file using FileSystem API

I am trying to create a file using the File system API..i googled and i get a code

function onFs(fs) {

  fs.root.getFile('log.txt', {create: true, exclusive: true},
      function(fileEntry) {
           fileEntry.getMetaData(function(md) {
            }, onError);

      },
      onError
  );
}

window.requestFileSystem(TEMPORARY, 1024*1024 /*1MB*/, onFs, onError);

can any one say what is the fs which is passed as function argument..

Please refer me a good example...