The Windows registry is a directory which stores settings and options for the operating system for Microsoft Windows 32-bit versions, 64-bit versions, and Windows Mobile. It contains information and settings for all the hardware, operating system software, most non-operating system software, users, preferences of the PC, etc. Whenever a user makes changes to Control Panel settings, file associations, system policies, or most installed software, the changes are reflected and stored in the registry. The registry also provides a window into the operation of the kernel, exposing runtime information such as performance counters and currently active hardware.
The registry is important because it stores vital information about your Windows system and its configuration, as well as info about all application programs installed on your computer.
Whenever you add and remove software, change your settings, add new devices, or upgrade your system, your registry is changed. For example, when you install a new printer, information about this device is written to the corresponding part of the registry. If this data is then somehow corrupted, you will not be able to work with the printer.