The following command works with all Ubuntu versions.
You should receive a result similar to the following:
Distributor ID: Ubuntu
Description: Ubuntu 14.04.1 LTS
The output can be interpreted with the following key:
14 – Year Released
04 – Month Released
1 – Minor Revision
LTS means “Long Term Support”. Every fourth release, in the second quarter of even-numbered years, is designated as such. This indicates they will be supported and receive updates for five years.