It's a bunch of predefined functions/commands, etc., and most (all?) are designed to work cross-browser. So it's basically a list of shortcuts. It's never required, because it's just based in regular JS itself, but it can make things faster, although the tradeoff is requiring the jquery library itself.
But basically you don't need to bother with it unless you want shortcuts or predefined routines. You also might need it if you want to use scripts that use it, of course.
No problem. And I don't mean to sound discouraging about it-- lots of people use it, but it's just not actually required. So if having shortcuts like that sounds useful, then it's a good idea. It also has some functions not included in regular JS but those are just combinations of existing functions (sometimes in useful ways).
jQuery is a great tool :)I`ve found a couple of articles on this topic, hope they will help you:
Those are some great links as to why jQuery is so good. One thing I'm not sure was mentioned is that it can play nice with other script libraries if need be. My favorite one, and this is mentioned by all familiar with jQuery, is that it allows you to write so much more script with much less actual code.