Adobe Photoshop CS5 Scripting Guide
Photoshop Object Model
A document object model (DOM) is an application programming interface (API), which allows you to programmatically access various components of a document (as defined for that application) through a scripting language. For additional information about Adobe object models and the scripting languages that support them, see Introduction to Scripting.
The Photoshop DOM consists of a hierarchical representation of the Photoshop application, the documents used in it, and the components of the documents. The DOM allows you to programmatically access and manipulate the document and its components. For example, through the DOM, you can create a new document, add a layer to an existing document, or change the background color of a layer. Most of the functionality available through the Photoshop user interface is available through the DOM.
A good understanding of the Photoshop DOM, and how each aspect of the DOM relates to the Photoshop application and its documents will make script writing easier.