A root kit is a set of tools used by an intruder after cracking a computer system. These tools can help the attacker maintain his or her access to the system and use it for malicious purposes. An attacker enters the victims computer through a security loop hole, like a weak password or a missing patch and then installs his favorite collection of tools which will provide him backdoor(s) to remotely access the cracked system and also mask the fact that the system is compromised.

Though not very prevalent currently other than an open source NT rootkit called Hacker Defender, some malware programs are reportedly using rootkit like mechanisms to hide in the bowels of Windows to evade detection and removal.

Here is what Kurt Dillard, Rootkit guru and program manager with Microsoft Solutions for Security says about Rootkits: