Once you point your web page to the jquery library you can start to write jquery code. So what is jquery? It is a script language that allows interaction, functionality as well as manipulate html and css. So you can use jquery to add or remove classes or html elements or change attributes about either.
There are many jquery plugin that you can install into your web page. So this is jquery and sometimes html/css that is written and bundled and all you need to do is put the files in the right place in your root directory and integrate the plugin code with your own.
Here are a couple of good jquery plugins:
Foundation: Modal, Carousel
Caroufredsel: Image Carousel
CDrops: Smooth scroll horizontal