Arguably, one of the most complex and lethal types of malicious code today is the "rootkit." As its name implies, this type of malware can gain "root" access, the highest privilege level in Unix systems, as well as deploy its "kit" of small, often malicious, executable packages. The code can provide continual and undetectable access into a machine.
Today's rootkits draw their power from having access to the kernel of the operating system. These "kernel-mode" rootkits run at the same lower level as all other trusted system processes, thus granting system control and providing effective ways to remain hidden.