El Capitan was released on September 30th, 2015. It was released as a free upgrade for previous OS’s all the way from Snow Leopard up. El Capitan has refined allot of features you may use everyday on your Mac. It keeps all of the built in apps with previous Mac OS’s such as Garage band, Mail, Facetime, or Maps but adds multiple features to use all of them more fluently.
Adding new full screen and swipe commands multiple pages are now easier to access or move fluidly between. For example they added a new Split View feature which allows you to go completely full screen blocking the menu bar and apps at the bottom but instead of just viewing one app you can view 2 split directly down the center of the screen. You can do this with any 2 apps you choose without having to manually resize either of the two. Then if you needed the desktop again it is just one of the new swipe commands away. They also have a new Mission control feature. With just one swipe you can see every window you have open arranged in a single layer with nothing stacked and nothing hidden. This adds another easy way to go from app to app or anything else you have open at this time. There are plenty of new additions along with added security definitions involved in upgrading as well.
You can upgrade from OS X Yosemite, OS X Mavericks, OS X Mountain Lion, OS X Lion, and OS X Snow Leopard