We're on version 14. There are too many improvements to simply list.
Try http://en.wikipedia.org/wiki/Adobe_Photoshop_version_history for a (still incomplete) list of the features added with each version.
Just download the trial and see for yourself.
Good advice from the previous posters, but I would like to point out two items:
Smart Objects and Smart Filters
Those have been significant improvements.
But you may want to consider whether you want the Cloud License that goes with Photoshop CC or a Perpetual License for Photoshop CS6.
The Cloud License seems to be perceived as odious by some …
And as you have not been a each-version-up-grader regular payments may not be in your interest at all.
If you download a trial version of CS6 be sure to install the CS6 updates, CS6 is not stable without them. Adobe introduce some nasty bugs in CS4 which have not been fixed todate. If you use Photoshop Automation be careful when using automated processes that add Adjustment layers. They may not always work as they should.
Somehow when I first searched Wikipedia it didn't pull up that link. Then you provided it; just what I was looking for! I also found an interesting older overview here ...
To cap it off, the c.pfaffenbichler and JJMack posts had excellent specific info -- Thank You.