OS X 10.11 was not a significant update to Apple's operating system, but it included a number of performance updates. It was the first version to include "Metal," a graphics technology developed by Apple that accelerates Core Animation and Core Graphics processing. Metal makes full use of the CPU and GPU, boost system-level rendering by up to 50 percent. El Capitan also added "System Integrity Protection" (SIP), which protects files and directories from unauthorized modification.
While El Capitan was primarily a performance update to OS X, it did include a few new features. For example, it introduced "split view" for windows, which snaps a window to half the screen's width when you press and hold the green zoom button in the title bar. It also introduced text formatting in the Notes app and improved syncing with Notes and other applications that support iCloud. OS X 10.11 also added support for numerous gestures in Mail, Messages, and Safari. These gestures, such as swiping back or forward with more than one finger can done using the trackpad on a laptop or an external input device, such as Apple's Magic Trackpad.
NOTE: El Capitan is a rock formation within Yosemite National Park in California. Apple likely chose this name for OS X 10.11 since El Capitan was designed to be a refinement to Yosemite (10.10), rather than a major update.
Updated: July 21, 2017