Starling is an ActionScript 3 2D framework developed on top of the Stage3D APIs (available on desktop in Flash Player 11 and Adobe AIR 3). Starling is mainly designed for game development, but could be used for many other use cases. Starling makes it possible to write fast GPU accelerated applications without having to touch the low-level Stage3D APIs.
Most Flash developers want to be able to leverage GPU acceleration (through Stage3D) without the need to write such higher-level frameworks and dig into the low-level Stage3D APIs. Starling is completely designed after the Flash Player APIs and abstracts the complexity of Stage3D (Molehill) and allows easy and intuitive programming for everyone.
Obviously Starling is for ActionScript 3 developers, especially those involved in 2D game development; of course you will need to have a basic understanding of ActionScript 3. By its design (lightweight, flexible and simple), Starling can be used also be used for other use cases like UI programming. That said, everything is designed to be as intuitive as possible, so any Java™ or .Net™ developer will get the hang of it quickly as well.