How to tell what version of Ubuntu you are running.

There are (as usual) multiple ways to do this.

You can run:
```
lsb_release -a
```
This will give output such as:
```
No LSB modules are available.

Distributor ID:	Ubuntu

Description:	Ubuntu 8.10

Release:	8.10

Codename:	intrepid
```
Linux Standard Base (LSB) is a project that aims to create standards in order to improve compatibility between Linux distributions. Therefore, running lsb_release on Debian, Ubuntu, Slackware, etc. should yield output in this common format.
Some people recommend viewing the file /etc/lsb_release but this is not specified by the LSB project and some versions of some distributions do not have this file.
You can also view the file /etc/issue.
The contents of this file are printed before the login prompt (for example, if you telnet/ssh into the system or do not boot into X). On a default Ubuntu setup, this file will contain the Ubuntu version information. I would rather remember the above method, however, since it is (or has been) fairly common practice to change the system information message displayed to users attempting to login remotely as a sort of security through obfuscation tactic.
GNOME System Monitor displays system version information. I suspect it grabs it from lsb_release output anyway.