What is a rootkit?
Rootkits are a form of malware with the main purpose to keep itself hidden on a system. It is a kit in the way that it often comes as a set of tools. Some parts are to break into the system, while others have the goal of overwriting existing binaries and intercept system calls in the kernel. All with the goal to avoid detection by scanning tools.
There is 1 tool collection available that covers rootkit: