It builds on several contributed code bases (nWidgets, Burstlib, f(m)), which is why we refer to it sometimes as a "unified" toolkit. Dojo aims to solve some long-standing historical problems with DHTML which prevented mass adoption of dynamic web application development.
Dojo does all of these things by layering capabilities onto a very small core which provides the package system and little else. When you write scripts with Dojo, you can include as little or as much of the available APIs as you need to suit your needs. Dojo provides multiple points of entry, interpreter independence, forward looking APIs, and focuses on reducing barriers to adoption.