Using javascript on the web used to be a chore: crossbrowser support required a deep knowledge of each of the main browsers’ javascript support.

Then libraries appeared and changed that: prototype.js, motools.js, ext.js and jquery.js (by a young talented guy named John Resig) to cite the most popular.

A few years later, jquery has pretty much won the battle, mainly because of its ease of use, a very flexible syntax and pluggable architecture. It allows designers with no skills in javascript to easily add animations, effects and interactivity to their web pages, whereas developers can develop jquery plugins to add extra functionality and distribute their plugins, quickly making jquery a tool of choice among teams of web designers and developers.

See jquery.com for more info.