hook.dll definition, relationships, removal:

hook.dll is a library file that contains malicious code, which implements main parasite functions. This file usually needs to be loaded by executables or other pest files. However, some threats inject hook.dll in the system, so it works as an independent component, for example, as a harmful web browser plugin or hidden system service.
The hook.dll file is installed and used by plenty different key logging software.
In most cases hook.dll is responsible for all the harmful actions the pest takes. Removing this file will immediately terminate the parasite or partially disable it. That is why the hook.dll file must be manually deleted. If you cannot erase it, consider using recommended spyware removers, which will get rid of hook.dll and associated parasites for you.
Please note that the hook.dll file actually may be a fully legitimate part of the operating system or legitimate software. Often parasites use files with unsuspicious names, but malicious functionality. You should always carefully check the file before deleting it. It may not be related with malware, but can be required by your essential programs to work properly.