OS X El Capitan was previewed for the first time at WWDC 2015 on June 8. Version number OS X 10.11 and codenamed Gala, El Capitan was released on September 30, 2015.
The real-world El Capitan is a geological feature of Yosemite National Park, so it makes sense that OS X El Capitan builds on the foundation Apple laid for OS X Yosemite, as well. It doesn't have a dramatic rework of the user interface like Yosemite did. Instead, Apple's made numerous refinements and improvements to workflow, performance and efficiency to make the Mac faster and easier to use than ever.
Enhancements in El Capitan include a new split-view for apps to make it easier to switch between two apps at once. An improved Mission Control makes it much easier to switch between open apps, and there's a handy way to find your mouse pointer on big Retina display screens simply by shaking it to make it grow. Spotlight now responds to natural language commands. Mail gains new data detectors to make it easier to take action on information people are sending you, an updated Notes app lets you drag clippings in from apps like Safari, Maps, Photos, Pages, Numbers and Keynote. Photos adds editing extensions to make it easier to do more with your images without having to leave the app. Safari adds the ability to mute noisy web pages, pin frequently visited sites for easy visiting, and better performance and efficiency than ever. Maps finally gets support for public transit systems. Metal for OS X promises dramatically improved graphics performance, not just for games but for any app that leverages OpenGL. A host of other performance and security enhancements rounds out El Capitan's offerings. What's more, if your Mac can run Yosemite, it'll be able to run El Capitan too — and just like Yosemite, El Capitan will be absolutely free.