With jQuery you dont have to think about creating xml objects ect ect, everything is done for you, but with straight up javascript ajax you need to program every single step of the ajax call.
jQuery (网站) is a javascript framework that makes working with the DOM easier by building lots of high level functionality that can be used to search and interact with the DOM. Part of the functionality of jQuery implements a high-level interface to do AJAX requests. jQuery implements this interface abstractly, shielding the developer from the complexity of multi-browser support in making the request.