Apple released Mac OS X “El Capitan” 10.11 on Wednesday, September 30th, 2015.
Rather than being chock full of new features, El Capitan adds polish and, in my experience with the initial public betas, stability.
Here is some good information and a list of a few new features from the folks over at The Mac Observer
As always, make absolutely sure you have a good backup of your hard drive before embarking on any software update, particularly a significant operating system version upgrade such as El Capitan. I use and recommend Carbon Copy Cloner from Bombich Software for backing up my computers.