Ubuntu difference between a LTS Release and Normal Release

There is a new release every 6 months (in April and October), with the version number being year.month

for example:  Ubuntu 16.04 was released in April 2016.

• Every two years, the April release is a Long Term Support version.
• All normal releases (13.04 and later) are only supported for 9 months.
• All LTS releases (12.04 and later) are supported for five years on both the desktop and the server.

Also,

The Desktop refers to the packages that are in the main and restricted repositories, these are the ones that have the little Ubuntu icon next to them in Synaptic or are marked as Supported in the Software-Centre respectively.

The Server packages are the ones in the “server-ship” and “supported-common” seeds