Windsor Geek
What is a rootkit?

A rootkit is a program or a program kit that masks the presence of malware in an operating system.

A rootkit for Windows systems is a program that embeds itself into a host system and typically hides the evidence of its presence by intercepting and modifying low-level operating system functions. Moreover it can hide the presence of particular processes, folders, files and registry keys. Some rootkits install their own drivers and services in the system which also remain invisible.