Current Stable Version + LTS (Long Term Support) Release
Ubuntu 14.04 Trusty Tahr > Download
Previous LTS (Long Term Support) Release
Ubuntu 12.04 Precise Pangolin > Download
(Scroll down or search the page for Ubuntu 12.04 download links)
Ubuntu release cycle is every 6 months or twice a year set at April (xx.04) and October (yy.10) on both Desktop and Server versions, and gets free updates for up to 18 months.
Long Term Support cycle is every 2 years, and supported up to 3 years for Desktop and 5 years for Server.
*For the average user, LTS versions are recommended for use for its stability and support.