I have had the conversation several times recently about jQuery, dojo, YUI etc.
I’ve come to the conclusion that the main differences are that things like dojo and YUI are frameworks (I differentiate by them having a packaging system) whereas jQuery, prototype etc. are libraries (Although jQ will roll the UI library).
This blog post summarizes it nicely. jQuery is for n00bs
I do love jQuery for quickly getting things done on a website and love to read their code and bugs to collect the oddities of various browsers, but for a packaged componentized system dojo wins for me.