When a Ubuntu release reaches "end of life" it will receive no further maintenance updates, not even critical security upgrades. It is highly recommended to upgrade to a recent version of Ubuntu at this point. Please check the upgrade instructions for more details.
For standard Ubuntu releases before 13.04, the main component is maintained for 18 months by Canonical and the Ubuntu community. From 13.04 onward, the support duration is 9 months. During that time, there will be security fixes and other critical updates.
For LTS (long term support) releases before 12.04, server packages are maintained for five years, desktop packages are supported for three years, and other packages in the main component are supported for 18 months. For 12.04 and later, both desktop and server packages are supported for five years.
The ubuntu-support-status command will print the exact status of your system.