What is a rootkit?

A rootkit is a program designed to take fundamental control of a computer system without authorisation by the system's owners and legitimate managers. It is intended to seize control of the operating system running on the hardware. In simpler terms, they are like trojan virus as well and deceives users into believing they are safe to run on their systems. Techniques used to accomplish this can include concealing running processes from monitoring programs, or hiding files or system data from the operating system.

Basically a rootkit is a set of tools to let the individual that infected your computer, take control of it. Sony was famous for it a couple of years ago,in order to prevent illegal use of their music cds and DVDs, they downloaded a rootkit to the user computer. They also used it to get info of the user, such as likes in music, websites visited etc. _José