Below are five (5) top level Windows Registry key names.
HKCR = HKEY_CLASSES_ROOT
HKCU = HKEY_CURRENT_USER
HKLM = HKEY_LOCAL_MACHINE
HKU = HKEY_USERS
HKCC = HKEY_CURRENT_CONFIG
Once you launch the regedit command, these key names will appear and will show sub key names once you drill down with the keyboard or mouse.