General Syntax:

GUI mode:
    REGEDIT.EXE
Import (merge) a .REG file:
    REGEDIT.EXE [ /L:system | /R:user ] [ /S ] importfile.REG
Export to a (.REG) file:
    REGEDIT.EXE [ /L:system | /R:user ] /E exportfile "registry_key"
Compress the registry (Windows 98 only):
    REGEDIT.EXE [ /L:system | /R:user ] /C

Parameters:

importfile.REG
    .REG file to be imported (or "merged")
exportfile.REG
    File name the information should be written to
"registry_key"
    Registry key to be exported, e.g. "HKEY_CLASSES\ROOT\*\shell"
/S
    Silent, i.e. hide confirmation box when importing files
/E
    Export registry file
/L:system
    Specify the location of the system.dat to use
/R:user
    Specify the location of the user.dat to use
/C
    Compress [filename] (Windows 98 only)