Question: What is a DLL? What is it doing in our computer? Why we need to register it? Is DLL registration a common process of a single command? Is this installed/registered while we are installing a software?

Answer: 
DLLs are dynamic-link libraries:

> 
> Dynamic-link library, or DLL, is
>  Microsoft's implementation of the
>  shared library concept in the
>  Microsoft Windows and OS/2 operating
>  systems. These libraries usually have
>  the file extension DLL, OCX (for
>  libraries containing ActiveX
>  controls), or DRV (for legacy system
>  drivers).
> 
> 
> 

See [Dynamic-link library](http://en.wikipedia.org/wiki/Dynamic-link_library) on Wikipedia.
A dll is a library of functions that can be used by other programs. Not all dll's need to be registered, that applies only to those that expose their fuctionality as [COM](http://en.wikipedia.org/wiki/Component_Object_Model) objects.

Dll's can come from many places; a good number of them make up Windows as such, others come as part of various software that you install. Microsoft Office, for instance, registers dlls that exposes COM interfaces that allows other applications to incorporate functionality from the Office applications.