Registry is the most important software on your computer. To put it simple, every time you install a program on your computer you install the .dll files that are required for that peticular program. These .dll files hold and store all the setting and components that make up that program. New windows also use registry to control things like icon images, user accounts, hardware being used, ports that are being used, etc... Since registry controls most of the computer and files behind the scene it also means having slight errors and unneeded registry files is not uncommon.
If you can tell these registry errors are all simple fixes, most of these registry issues are unused file extentions, unused icons and .dlls for programs I no longer have. simple registry fixes like these can be found and fixed using programs like CCleaner.
Microsoft has also made registry data avaliable to edit. Personally, I would stay away from registry due to the potential to destroy the foundation of your computer. Editing the computer's direct registry allows you make tweaks and changes possibly for the better, but again unless you have expierence in tampering with your computer's registry I highly recommend not tampering with it.
To wrap this up we have learned that registry is defined as files on your computer that control and keeps the data of all your computer and computer programs. Changing and tampering with your registry without expierence can potentially damage the framework of your computer.