From the Releases page on the wiki:
Ubuntu releases are supported for 18
months. Ubuntu LTS (Long Term Support)
releases are supported for 3 years on
the desktop, and 5 years on the
This means that normal releases are will have bugfix and security updates for 18 months, while LTS releases are maintained for 3 or 5 years (depending on the version).
In other words: if you don't want to upgrade your system at least every 18 months, you'll want to use a LTS release so you'll be able to get security updates for a longer period.
As of the 12.04 LTS release, there is no distinction between server and desktop releases. LTS support is for 5 years despite release type. There is no longer any 3 year support. For a visualization of the support coverage see the Ubuntu wiki page about LTS: https://wiki.ubuntu.com/LTS.