Soon after setting up your server you if you check the logs you will notice some malicious behavior like probing for security holes in the system. Fail2Ban monitors log files for this behavior. It then typically updates the firewall rules to block the malicious IP address although it can be configured to perform other actions like sending email. To install fail2ban
sudo apt-get install fail2ban
The default configuration should be fine for now on most home web servers but you may want to look up more details on www.fail2ban.org