Hooking is one of the techniques used by a rootkit to alter the normal execution path of the operating system. Rootkit hooks are bascially installed modules which intercept the principal system services that all programs and the OS rely on. By using a hook, a rootkit can alter the information that the original OS function would have returned. There are many tables in an OS that can be hooked by a rootkit and those hooks are undetectable unless you know exactly what you're looking for.
Kernel hooks are not always bad since some system monitoring software and security tools use them
as well. If no hooks are active on your system it means that all system services are handled by ntoskrnl.exe which is a base component of Windows operating systems and the process used in the boot-up cycle of a computer.
For more info on this see Windows Rootkit Overview
The newer algorithms used by rootkit detectors, such as BlackLight, attempt to find what the rootkit is hiding instead of detecting the presence of the rootkits hooks.