This hand is placed release a complete set of tools for programming and development of applications in OS X for Mac, iPhone, iPad, and Apple Watch. This release is compatible only with the OS X Yosemite 10.10.x and OS X El Capitan 10.11.
To develop applications targeted to the previous version of OS X or iOS, refer to the resource materials developer.apple.com or - in the section "About SDKs and the iOS Simulator" from the menu item Xcode

Help> What's New
Included iOS SDK: iOS 9
Included Mac SDK: OS X v10.11; v10.10
Included Apple Watch SDK: watchOS 2.0
This release is available as a solid application.
To install Xcode - double-click the downloaded DMG file and drag the file to a folder Xcode.app Applications.
When running Xcode has the ability to run additional development tools such as the Instruments or the FileMerge, via the menu item

Xcode> Open Developer Tool
This will allow you to continue working with additional development tools directly from the Dock for easy access even when Xcode is completed.

Xcode includes everything developers need to create great applications for Mac, iPhone, iPad, and Apple Watch.
Xcode provides developers a unified workflow for user interface design, coding, testing, and debugging.
The Xcode IDE combined with the Cocoa frameworks and Swift programming language make developing apps easier and more fun than ever before.