jQuery is a popular library for manipulating the DOM and executing Ajax requests. There’s nothing you can do with jQuery that you can’t do with the DOM API (jQuery is, after all, built on the DOM API itself), but it offers three main advantages:
jQuery protects you from idiosyncrasies in the way different browsers implement the DOM API (especially older browsers).
jQuery provides a simpler Ajax API (which is welcome because Ajax is used heavily in today’s websites).
jQuery provides many powerful and compact enhancements on the built-in DOM API.