A rootkit is a general description of a set of programs which work to subvert control of an operating system from its legitimate operators. Usually, a rootkit will obscure its installation and attempt to prevent its removal through a subversion of standard system security. Techniques used to accomplish this can include concealing running processes, files or system data from the operating system.[1] Rootkits have their origin in benign applications, but in recent years have been used increasingly by malware to help intruders maintain access to systems while avoiding detection. Rootkits exist for a variety of operating systems, such as Microsoft Windows, Mac OS X[2] [3] , Linux and Solaris. Rootkits often modify parts of the operating system or install themselves as drivers or kernel modules.(Wikipedia)

A rootkit allows someone, either legitimate or malicious, to maintain command and control over a computer system, without the the computer system user knowing about it. This means that the owner of the rootkit is capable of executing files and changing system configurations on the target machine, as well as accessing log files or monitoring activity to covertly spy on the user's computer usage.About.com

AVG has a free program that will detect Rootkit's. AVG Anti-Rootkit is a powerful tool with state-of-the-art technology for detection and removal of rootkits. Rootkits are used to hide the presence of a malicious object like trojans or keyloggers on your computer. If a threat uses rootkit technology to hide itself it is very hard to find the malware on your PC. AVG Anti-Rootkit gives you the power to find and delete the rootkit and to uncover the threat the rootkit is hiding. AVG Rootkit