This is a question that I ask myself a lot and keep having to look up almost every time. Sometimes I am not certain what distribution I am actually using. Here is the summery version that hopefully will save me having to look the command up each time, and perhaps help you as well.
To find out all the version information about the Linux kernel you are running run the command…
uname --all
To find out the OS release (such as the Ubuntu version) have a look in the os-release file…
more /etc/os-release
…and on most systems you can look at that same information neatly formatted using…
Delicious
Flickr
Google Plus
Hackaday
Instructables
Lanyrd
Twitter
YouTube
Facebook
Geocaching
Useful commands. Thanks.