What it is

JQuery is an extremely popular open source javascript library whose main purpose is to wrap browser APIs like the DOM API with more convenient syntax in order to make them easier to use.

For instance, instead of writing the verbose and cumbersome:

var p = document.createElement("p");
p.appendChild(document.createTextNode("more content");

In JQuery, you can write:

$("#container").append("<p>more content</p>);
// the JQuery API can be accessed from the “JQuery” object or the shortcut “$”.

In addition to DOM access and manipulation, JQuery provides useful methods for event handling, AJAX, animations and more.

Why it matters

Historically, JQuery was important during the browser wars because browser vendors implemented the DOM API standard inconsistently, and JQuery was the best way to ensure consistent behavior across them. That is less relevant nowadays as browsers gradually converge.

Today, It is arguable whether you should be using JQuery in your projects. On one hand, it makes development easier, but it also requires downloading a large javascript library on your site which can slow down page load times. It is good to know JQuery because you are bound to run into it, but my personal opinion is that if you should avoid it when you don’t absolutely need it.

How to learn it

Here is the official JQuery API documentation

You can get started at learn.JQuery.com.