Exploit, in computer security, is technique or program to leverage the vulnerability in computer or software, that would cause the system to not work as expected. It could caused the system to output or display something that is not expected, and in worst case can cause data breach or system down.
Metasploit is one of the top tools for pen-testing. It has a framework that contains a lot of exploits, and you can write your own exploits too.
I actually planned FQDN for today. But... maybe when I don't have any term in mind next time. :P