Dojo has one peculiarity compared to most other Ajax frameworks: it needs its entire src/ catalog. That means that you can't (like with many other popular Ajax toolkits or frameworks) just copy the base file, dojo.js, to your web directory, include it, and call it a day you must copy the whole distribution, which is several megabytes.
Dojo consists of a base package, which can be zipped down to just over 25K, and contains a lot of nice features, like the CSS query engine, event management, and so on.
The rest of Dojo's functionality — the Dijit widget system, the visionary and cutting-edge dojox stuff, and everything else, is loaded as required either by the web page or by the components themselves.
When you download a pack of Dojo, you will see a directory structure looking a bit like this:
dojo dijit dojox util
The util directory doesn't contain anything that you'll access from your web page, but has some nifty things. It has build scripts (using Java) that can build...