The Microsoft Computer Dictionary, Fifth Edition, defines the registry as:
A central hierarchical database used in Microsoft Windows 98, Windows CE, Windows NT, and Windows 2000 used to store information that is necessary to configure the system for one or more users, applications and hardware devices.

For more brief defination of registry, hives, keys, subkeys & values, read microsoft's help at http://support.microsoft.com/kb/256986 & http://en.wikipedia.org/wiki/Windows_Registry