The term rootkit is used to define a Trojan (or technology) used to hide the presence of a malicious object (process, file, registry key, network port) from the computer user or administrator.
Sony was in the midst of a controversy earlier for the same reason. Rootkits are very dangerous as they are not visible to the naked eye and can be used to steal informatiom. Now Sophos is providing a free rootkit scanner (which is a component of Sophos Anti Virus) to detect and remove rootkits.