Dojo is a set of layered libraries. The bottom most layer is the packaging system that enables you to customize the distribution of Dojo for your application. On top of Dojo's package system reside language libraries such as the Dojo event system and the language utilities that greatly improve and simplify the lives of JavaScript developers. Environment-specific libraries are provided in some cases, but in most uses, you'll only need to know that all of Dojo works without incident across every major browser.
The bulk of the Dojo code lives in the application support libraries, which are too numerous to display completely in the diagram. dojo.gfx provides native vector graphics support across the major browser implementations of SVG and VML. dojo.lfx is a lightweight effects library, while dojo.io is "where the ajax lives."
Most of the "action" in Dojo is in the widget toolkit, that contains a template-driven system for constructing widgets, a declarative parser for putting widgets in pages without writing JavaScript, and a set of base components that implement common-case solutions to web interaction problems that HTML+CSS alone cannot solve.