There is no universal way to describe a user interface. It's typical for a designer to hand over image assets as one or a combination of these things:
- Individual files. Probably 32bit PNGs if you're creating an iOS, Android or Mac app.
- Sprite sheets. More common for games than apps. Sprite sheets are also known as texture atlases.
- Photoshop documents (PSD). Sometimes developers prefer to cut things up themselves, so they can experiment with different ways to implement the UI in code.
- Some information describing the non-image aspects of the design. Text size and colour etc.
Is the goal final production assets? If so, chances are the developer will (or should) want things as described above. If you're just creating mock ups to demonstrate how the app works, then there's many approaches, including using HTML/CSS, Apple's Keynote app. Basically anything that can demonstrate how the app moves and fits together.
Ultimately, the decision on how to construct the UI is with the developer. Even if it's an iOS app, where the IDE is pretty locked down, there's still many ways of approaching the code side (XIBs, auto-layout, doing everything in code).