The Microsoft Windows registry is a file (some people describe it as a hierarchical database, and that is not a completely inaccurate description because of how the file is laid out) that stores configuration information for the Windows Operating System, as well as many of the software programs installed on the system. All of the information inside the Registry can be manipulated with a variety of tools to further customize how Windows looks, how it works, or the software on it. But this is not without substantial risk. Changing application (or system configurations) outside of appropriate tools (like the Control Panel, as one example) can cause a lot of problems. You may not even be aware of the impact until much later (the next time you use a program - fater a reboot, for example). I want to stress this - Editing the Registry is very risky and can cause many problems if you don't know what you're doing or edit the wrong things - to the point where Windows may no longer function.
The two most common tools for editing the registry on Windows are RegEdt32.exe and RegEdit.exe. However, there are other tools available and Yannis Grammatis identifies them on his website http://www.chaminade.org/MIS/Articles/RegistryEdit.htm.
Again, I want to reinforce my earlier warning.....
Editing the Registry is very risky and can cause many problems if you don't know what you're doing or edit the wrong things - to the point where Windows may no longer function.
So with these notes, why would you want to edit the registry? For some people it's a way to tweak (modify some obscure setting) that you normally wouldn't be able to access. Other settings may expose functionality for a program or feature you wouldn't normally be able to access - for instance a specific mouse may not fully emulate a different mouse - but by tweaking the setting in the registry you can convince Windows that the feature is supported.
This is not something I normally recommend doing - it is only for advanced users or people who don't worry about "breaking" their computer.