The short answer is that jQuery is primarily a JavaScript library for doing DOM manipulation, Ajax, events, and effects. Dojo is a more comprehensive toolkit that offers those same features, but also includes many other pieces that facilitate building rich web applications, such as patterns for code organization, tools for abstracting data syncing between server and browser, internationalization, accessibility, templated widgets, graphing and charting, rich UI widgets, and much more. Dojo also provides dependency management and build/optimization tools for creating a production-ready JavaScript application.
