JQuery is a pre-packaged, pre-tested javascript api which will greatly improve the lives of any devs having to deal with javascript in any way. It allows for easy access to dom elements, for example:

Before

var someId = document.getElementById('someId');

After

var someId = $('someId');

Another example in Jquery’s ease of use is hiding/displaying dom elements, for example:

Before

document.getElementById('someId').style.display = 'none';
document.getEelmentById('someId').style.display = 'block';

After

$('someId').hide();
$('someId').show();

As you can see, JQuery greatly reduces the amount of work needed to accomplish seemingly easy tasks. If that isn’t enough to sell you on it as being a great api, consider this; JQuery is also cross-browser tested! So you can rest assured that whatever javascript you implement on your projects will work in most currently supported browsers.

Visit JQuerys website for more info.