Lightweight Footprint, CSS3 Compliant, cross-browser.
Regarding the documentation - it really makes sense to grab a book in order to get started.
jQuery is made to make difficult things very simple. It keeps you away from writing cross-browser compatabile code, implementing simple aniamtions or recursive methods to update multiple dom-elements. It helps you get things done very quickly and saves you a lot of frustration.
jQuery is a big project with huge community so it’s very good tested and you don’t have problems with finding solutions to your problems.