Rootkit

A rootkit is a computer virus which allows someone to take control on your computer. It is very dangerous.

It is a stealthy type of software, typically malicious, designed to hide the existence of certain processes or programs from normal methods of detection. It allows continued secret access to a computer.^[1]

Rootkits can be a single piece of code or multiple tools working together to affect the targets computer.

Rootkit Media

Computer security rings (Note that Ring -1 is not shown)
The rkhunter utility uses SHA-1 hashes to verify the integrity of system files.

References

↑ Rootkits, Part 1 of 3: The growing threat. McAfee 2006. [1]

This short article about technology can be made longer. You can help Wikipedia by adding to it.

[McAfee1-1] Rootkits, Part 1 of 3: The growing threat. McAfee 2006. [1]

[1]