jQuery is one of these libraries.
With jQuery, I can add animations, control contact form entries and a whole host of other pieces of magic in your website.
Take the DataMouse.biz website. jQuery is used in several areas of the site to improve your browsing experience without you really noticing:
- the sliding contact form at the top of every page
- the home page rotating images
- the services pages, with their animations
- pop-up comments when you hover over certain elements, like the Facebook link at the top of the page
All of these small effects add to your experience of the site, and should be taken for granted as a sign of great design.