Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Ubuntu GNU/Linux Long Term Support which are the Ubuntu GNU/Linux versions that are supported for three years for desktop versions and five years for server versions.
The first LTS Ubuntu version that was issued was Ubuntu 6.06 LTS Dapper Drake, which was released in June 2006. The second was Ubuntu 8.04 LTS Hardy Heron, which was released in April 2004 (per the numbering scheme -- first digit being the issuance year and second being the month number.) The next scheduled LTS Ubuntu version is Ubuntu 10.04 (which is currently tentatively scheduled.)
As may be already evident, there is no true way to get the full three years (much less five) of support (which is actually just operating system and software security and stability updates -- basically bug fixes.) You see if these versions are supported for three years but are issued every two years, how is it possible to get continuously three full years or supported use without the need to do a full reinstall or upgrade of your operating system.