Current Stable Version
Ubuntu 13.04 Raring Ringtail > Download
Latest LTS (Long Term Support) Release
Ubuntu 12.04 Precise Pangolin > Download
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.