What is El Capitan?
In OS X El Capitan, there have been a number of upgrades that improve both performance, and overall user experience. For example, the spotlight function has been revamped and now possesses natural language input, as opposed to the keyword function it uses now (shown below).
Furthermore, changes such as better window management, and new aesthetic changes to existing Apple functions have been implemented. Safari also has a new "pinned site" function, which will enable you to surf the web even more efficiently.
In addition to that, Apple has made a number of performance upgrades, such as, 1.4x faster app launch speed, 2x faster app switching, 2x faster display of mail and 4x faster PDF opening in Preview .
For more detailed descriptions of the improvements to OS X, visit the links at the bottom of the page.