This article talks about detecting rootkits in Linux.
A rootkit is a set of software tools intended to hide running processes, files or system data from the operating system and ultimately from real root user. They are used by crackers as well as commercial companies for copy protection (Sony BMG CD).
However rootkit is not just for Linux, they exist for Windows and other operating system too.
From the article:
A rootkit is a collection of tools a hacker installs on a victim computer after gaining initial access. It generally consists of network sniffers, log-cleaning scripts, and trojaned replacements of core system utilities such as ps, netstat, ifconfig, and killall.