Windows Registry is a critical component of your Windows operating system that governs the operation of Windows. Without a clean and well-organized registry, your Windows system is vulnerable to corruption, freezing, and crashing.
Information about file extensions and how they are mapped to applications that open these files is also stored in the registry. In fact it's one of the largest sections of it. When this data gets corrupted files may fail to open making you music, documents or photos inaccessible.
Windows Registry is stored in several files on the main system partition for performance and redundancy reasons. Registry consists of "hives" that group information together. Hives consists of keys that can contain subkeys and values much like file system folders can contain subfolders and files. Keys, subkeys and values have names like HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Services\ adpahci\ Type".
While Windows has special mechanisms in place to preserve low-level consistency of the registry it does little to preserve the high level consistency. If the registry was a box of scribble pieces Windows would make sure that none a broken or lost and that you can read the letters but it would care little for them to be arranged in a meaningful way and form words.
Registry is modified as you install and uninstall software or hardware, change computer settings or browse the Internet. Some of these modifications cause errors. Because Windows does such a good job at preserving the registry these errors persist and accumulate over time. These errors may cause computer crashes, slower performance, programs not responsing completely and errors when trying to open some of the files. Eventually too many errors may cause computer to crash completely in such a way that operating system needs to be reinstalled. Often when this happens hard drive is also formatted which causes all information to be deleted.
This is where registry cleaners come in. Registry cleaner is an application that needs to be run on a regular basis to check for errors in the registry an repair them. A good registry cleaner needs only several minutes to scan through the entire registry on a modern computer and apply different algorithms repair the errors.