In the Windows 98, CE, NT, and 2000 operating systems, a registry key is an organizational unit in the Windows registry, an internal database the computer uses to store configuration information. The registry in Windows stores data in binary format, keeping the configuration data for the machine and its users in separate files. This allows the system and its applications to load global and individual configurations upon startup and login.
When an administrator runs the command regedit, pre-defined keys called root keys, high-level keys or HKEYS display in the left pane of the Registry Editor window. To a lay person, a registry key looks just like any other Windows folder. A pre-defined key and its nested subkeys are collectively called a hive.
An application must open a key before it can add data to the registry, so having pre-defined keys that are always open helps an application navigate the registry. Although pre-defined keys cannot be changed, subkeys can be modified or deleted as long as the user has permission to do so and the subkey is not located directly under a high-level key.
Before making any changes to registry keys, however, Microsoft strongly recommends the registry be backed up and that the end user only change values in the registry that they understand or have been told to change by a trusted advisor. Keys and subkeys are referred to with a syntax that's similar to Windows' path names, using backslashes to indicate levels in the hierarchy. Edits to the registry that cause syntax errors can make the computer inoperable.