Spark Architecture Overview

Here's a very nice post on Spark Architecture

Here's another great piece on Spark
Flex 4's new MXML subset for skins, FGX, is valid XML code, and can be specified either inside an MXML file defining a component or, more often, in a separate file.

The advantage of using a separate file is that designer tools can read and write that file directly.

Adobe, for instance, enhanced its designer tools to comprehend such FGX files, and even provided a specialized designer tool, Catalyst, for the purpose of working with FGX content.

FGX files can declare any visual aspect of a component. An FGX file, for instance, can define runtime graphics classes that draw shapes, specify containment of sub-components, define component effects, transitions, and even specify layout inside the component. In Flex 4, the runtime graphics primitives directly map to graphics classes in Flash Player 10.

Separating out visual concerns into a separate file allows a Flex component's core class to focus on component logic, such as responding to user gestures and data events.
A new component lifecycle ensures that the component's core behavior is associated with the right skin file at runtime.