Every pc user knows about computer viruses & people are rightly fearful about them. Many users have also know about (computer) worms, which are nasty scripts designed to spread as much as they can to attack computers.
On the other hand, a rootkit is in a different way. This unwanted code on your computer is used to gain control over your desktop by residing deep inside your operating system. Unlike most viruses, it is not directly destructive and unlike worms, its objective is not to spread infection as wide as possible.
So what does a Rookit do? In this video I tried to explain few details about rootkits which became challenges to antivirus security softwares.