Definition: A "Root Kit" is a particularly vicious threat vector. The malicious code's main attribute is its capability to interfere with the administrative level of a computer or network's operating system and is likely to be more invisible from normal examination. The presence of a root kit gives an attacker administrative privileges. The attacker is likely to be able to remain concealed while still initiating operations in your computer about which you are unaware. Once a system or network in compromised by such malware access can be given to other attackers.
Its Relevance: Nefarious software that can gain administrative level access to information infrastructure is particularly insidious. The attack can continue for an unlimited amount of time and be expanded upon. Unknown to the user, for example, every key stroke that is entered could be forwarded to the intruder.