HKLM stands for the HKEY_LOCAL_MACHINE key in the Windows Registry and is used to store system configuration information. This includes settings like computer and network configurations, user account profile information, installed software, and hardware information. The settings stored in the HKLM can be accessed by all users on a computer or domain.
HKCU stands for the HKEY_CURRENT_USER key in the Windows Registry and is used to store settings related to the currently logged-in user. This includes settings like language preferences, desktop wallpaper and display settings, user emoticons, mouse and keyboard settings, window layout and size, and more. As the name implies, only the user that is currently logged in can access the settings stored in the HKCU.
The difference between the two keys is the scope of their settings. The HKLM stores settings that are global and can be accessed by all users, while the HKCU stores settings that are specific to each individual user. System administrators will generally make changes to the HKLM when configuring settings for all users, while each user should be able to configure their own settings through the HKCU.