A registry item that can contain subkeys and values. Keys are used for organizing the registry into a hierarchical structure and for naming and identifying collections of registry values.
Keys are thus analogous to directories in a file system as displayed using Microsoft Windows Explorer, and keys are in fact displayed by the Registry Editor as folders. The screen capture shows the Control key being displayed, which contains value entries (in the right pane) and subkeys (the series of items under it in the left pane). The Control key itself is a subkey of the CurrentControlSet key.
Graphic K-3. The Control key in the Windows NT and Windows 2000 RegistryEditor (regedt32.exe).