Alloy is an MVC framework for building mobile apps with Titanium. It uses XML for UI markup and allows apps to be highly customizable. Apps are compiled into CommonJS modules for cross-platform use on iOS and Android. The framework includes features like views, controllers, models, styles, assets, and widgets to build apps. It also provides error checking during compilation to catch issues early. Developers can get started quickly with Alloy through tutorials and documentation available on its website.