The Windows Registry
So what is Windows registry? To be short, it's a unified store of all system settings - a complex database. Registry possesses ierarchical structure as a tree. Registry holds information about hardware, software, user profiles and other vital data.
Registry was introduced in Windows 95 to remove discomfort with .ini files that Microsoft developers created in earlier Windows 3.1.
So since Windows 95 life became better. And though appearance of the registry was undoubtedly a big step ahead, it brought some problems which need attention.
One of the problems is that ordinary users happen to edit some registry keys/values and some of those registry "tunings" may easily cause the system to become unbootable or even worser, cause you may get some weird bugs from time to time. Registry is a fragile thing and if you're not careful you can easily damage it.
Another problem is that registry can only grow in size. Advanced users and system administrators have to modify registry entries frequently. But all those changes made either by the user (manually) or by the program cause registry to grow and lose its linear structure. The loss of the linear structure in its turn slows down your system. To prevent such unwanted decelerations the registry must be constantly defragmented in order to keep up to its linear structure.
I've already mentioned that Windows registry is a hierarchical database, but let me say some words more. The root of registry hierarchy is called My Computer. The root in its turn possesses root keys which can be compared to subdirectories and they. These root keys may contain subkeys (which again can contain farther subkeys and values) and values. Each value has its name (any characters not those in ["\","*","?"]), type and data. These entities are called root handles, which is quite understandable. But they are also called hives, which is not reasonable, but you should remember what it is.