Rootkit: “A rootkit is a stealthy type of malicious software designed to hide the existence of certain processes or programs from normal methods of detection and enables continued privileged access to a computer.” – wikipedia
A great tool to check for rootkit presence is chkrootkit tool.
Chkrootkit scans locally for signs of a rootkit.
To install chkrootkit (for Ubuntu users):
sudo apt-get install chkrootkit
To run chkrootkit, issue the following command: