Xcode is the most important tools required for developing apps on OS X Yosemite, this one looks like will be the final version Xcode 6.1 build number 6A1042b
This pre-release version of the Xcode 6.1 developer tools is required when developing for OS X Yosemite, and includes the Swift programming language.
Xcode 6.1 GM Seed runs on OS X Mavericks and OS X Yosemite
This is the developer toolset for Mac, iPhone, and iPad. It includes the Xcode IDE, iOS Simulator, and all required tools and frameworks for building OS X and iOS apps.
What's New in Xcode from the Help > What's New in Xcode command when running Xcode.
Included iOS SDK: iOS 8.1 beta
Included Mac SDK: OS X v10.10
This release is distributed as a single application bundle.
To install, double-click the DMG file, and drag the Xcode.app file to your Applications folder.
Within Xcode you can launch additional developer tools, such as Instruments and FileMerge, via the menu item
Xcode > Open Developer Tool
You can then keep the additional tools in your Dock for access when Xcode is not running.