iOS 9 is Apple's newest operating system for iOS devices like the iPhone and the iPad, introduced at the Worldwide Developers Conference on June 8. iOS 9 builds on the content introduced with iOS 7 and iOS 8, bringing subtle design changes, refined features, improved functionality, and performance enhancements.
OS X 10.11 El Capitan, introduced at the Worldwide Developers Conference on June 8, is the next iteration of OS X, building on the features and design changes introduced with OS X Yosemite. OS X El Capitan might seem like a strange name, but it's meant to highlight the OS's position as an update that brings under-the-hood improvements and refinements to OS X Yosemite.
In real life, El Capitan is one of the most popular rock formations and landmarks located within Yosemite National Park. The "El Capitan" name for OS X 10.11 reflects a long-running OS X naming scheme that's used to denote updates that are refinements to previous updates, following in the footsteps of Leopard/Snow Leopard and Lion/Mountain Lion.
Source and credit: macrumors.com