Rootkit
A rootkit is a computer software or a collection of computer
softwares which is designed to hide the fact that the user's computer
has been compromised.
The rootkit is installed side by side to other malware, that benefits
the intruder.
Once the PC is infected by a type of malware the rootkit
will take action and cover the tracks of the malware
making it possible for the intruder to access the infected PC easily.