Anyhow there are many ways that you can find the current version of your Ubuntu OS but I'll do my best to show how to get it by using two or perhaps three ways so someone new to Ubuntu would be able to retrieve that information in the future (hopefully).
The classical way...
Remember, this to work you have to use the Gnome classic desktop and with the upcoming Ubuntu 11.10 it won't be there by default, so in the future, unless you've manually installed Gnome, this option may not be available.
*. Anyhow, if you're ready, after log-in to Gnome, from the main application menu (as in the below screenshot), go to: "System" - "About Ubuntu".
*. After clicking on that, it should bring a new window similar to the one below, the numbers will differ according to your actual Ubuntu version (11.04 in this example).
|I've highlighted the text for you to locate easily...|
This however I couldn't find in the new Unity desktop. Although by default Unity does not come with a classic menu but even after using the classic menu indicator, I couldn't find it.
But luckily we can still find the current Ubuntu version by using a simple trick. Click on an empty area on your desktop (this works in all desktops - Gnome, KDE, Unity, etc) to make sure the desktop is selected.
Now press the "F2" key on your Keyboard which should bring you the official Ubuntu help page (in your HDD, so no Internet connection is necessary) and as you can see with the below screenshot, you should see the version at the top of the page.
Universal method (2), if all fail ... this should and WILL come to rescue you! :).
Now, GNU/Linux is universal, but unlike any other time, now we have dozens or hundreds of different distributions. Some are "just" derivatives but we do have others that are original or derivatives still have done a lot of customizations of their own thus they're almost original.
After seeing this, the Linux Foundation became concerned that GNU/Linux as a whole and thought in the future it could loose its "identity" easily. So they installed a standardization called LSB which stands for: Linux Standard Base.
This basically ensures that no matter how many customizations that the GNU/Linux system (Kernel, desktop, etc) undergoes under different developers, still some of the basic/core functions (commands and protocols) will be implemented universally. I wouldn't go into details since its really boring (not an expert on the matter either to be honest) as hell and out of the subject as well :).
Anyhow, thanks to this "move" there's a command that'll always exist no matter what changes occur that'll let you find not just the the version of the Ubuntu OS but any GNU/Linux distribution in general, any given time!. So to use that just open your Terminal and enter the below command.
lsb_release -r -i -d -cBelow is my output. As said, if all fail, then this should work for you dudes and dugees in your search for beginning to learn GNU/Linux (oh well, we all are anyway :D, at least I am :/).
The command comes with a simple documentation which you can access by using the below command. Enjoy!.