The registry is split into five different sections, two for the current user, two for the machine, and one for system startup.
HKEY_CLASSES_ROOT is a subkey of HKEY_LOCAL_MACHINE\Software. This section stores all of the information for registered applications, including file associations. This key is sometimes abbreviated as HKCR. If you write a registry key to HKEY_CLASSES_ROOT and the key already exists under HKEY_CURRENT_USER\Software\Classes, Windows will use the information stored in the HKEY_CURRENT_USER\Software\Classes as the master key instead.
The HKEY_CURRENT_USER section contains configuration options for the current user’s profile, including the location pointers to the user’s documents folders, display options, and Control Panel settings. This registry key is sometimes referred to as HKCU.
The HKEY_LOCAL_MACHINE section of registry keys is the most commonly altered. It contains settings specific to the PC, including installed applications. It is sometime known as HKLM.
The HKEY_USERS section in the registry contains keys that are subkeys corresponding to HKEY_CURRENT_USER. It stores information for each user profile that is actively loaded. It is sometimes known as HKU.
HKEY_CURRENT_CONFIG includes information gathered at startup. Information and keys are stored here temporarily and are replaced when the PC is restarted.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out