Ubuntu 13.10 (Saucy Salamander) reached end of life today. That means that if you use Ubuntu 13.10, you should upgrade to Ubuntu 14.04 LTS (Long Term Support). That's because after July 17, 2014, "Ubuntu Security Notices will no longer include information or updated packages for Ubuntu 13.10".
It's also important to mention that PPA maintainers will no longer be able to upload packages for Ubuntu 13.10 soon.
Ubuntu 13.10 was the second release which had a 9 month support cycle, after Ubuntu 13.04. Previously, non-LTS releases were supported for 18 months, that's why, for example, Ubuntu 13.04 reached end of life before Ubuntu 12.10.
As a reminder, the only supported Ubuntu releases are now: Ubuntu 14.04 LTS (supported until April 2019), Ubuntu 12.04 LTS (supported until April 2017) and Ubuntu 10.04 LTS for servers (supported until April 2015; Ubuntu 10.04 for desktop reached end of life on May 9, 2013). For more information, see the Ubuntu Releases wiki page.