HKU
Hive Key User
SID Sub-key for all loaded User Profiles
HKLM
Hive Key Local Machine
Contains specific information about the hardware, software, and preferences for all users who log onto the system.
HKCR
HKey Classes Root
Used to associate file types with programs that are used to open them
HKCU
HKey Current User
Contains user profile environment settings of the interactively logged on user
HKCC
HKey Current Config
used to establish the current hardware configuration profile