Rootkit is an application, that hides its presence or presence of another application (virus, spyware, etc.) on the computer, disguising themselves as operating system files which makes them almost undetectable by common anti-malware software. Please note that rootkits can be either correct or malicious. Correct rootkits may be installed as a part of legitimate application.
What does a Rootkit do?
A malicious rootkit provides unauthorized access and administrator priviliges of your computer's system and your data to a remote user. This is done with the intention of covertly stealing user passwords, credit card information, computing resources, or conducting other unauthorized activities.