To launch the explorer in the directory of the file currently edited:
   :Explore
If the file has changes the window is split.  To always split the window:
   :Sexplore
To launch the explorer in a specific directory:
   :Explore dirname
   :Sexplore dirname

From inside the explorer move your cursor to a line containing a file or
directory name.  The following command keys are available:

  <enter> will open the file in the window the explorer is currently
      occupying.
  'o' will split a new window and open the file in the new window.
  'O' will open the file chosen using the window that the cursor was in just
      before you started or entered the explorer window.  If the explorer is
      the only window, it will first split a new window to use for the file to
      be opened.
  'p' will open (or use) the preview window showing the file
  'x' will execute the file with the system tools.  Only when supported
      (currently MS-Windows and KDE).