Rkhunter is an easy to use tool that keeps an eye on your systems files for rootkits and a few other oddities that may indicate an attacker has taken over your system. It can be also used if your system has been hacked and you wish to inspect the damage - keep in mind that it if you have been compromised it should be run off a static medium such as a live cd that will allow you to check your system with confidence that the rkhunter binaries have not been compromised.
Rootkits are hidden and modified programs that an attacker will hide within your system allowing them full monitoring and access via backdoors, hidden accounts and plenty more.
Installing and usage in Ubuntu (Jaunty 9.04) is very simple.
apt-get install rkhunter
To update: rkhunter --update
To check: rkhunter --check
apt-get install rkhunter To update: rkhunter --update To check: rkhunter --check