Which version of Ubuntu am I running?

You should usually know the version of Ubuntu you are running, but in case you can not determine the version you are running, just run this command.

cat /etc/issue

That command will tell you the version of Ubuntu you are using.

[Update]

Somebody sent me an email telling me that this commands are a lot better.

lsb_release -a

or

lsb_release -rcs