What is jQuery?
jQuery is a JavaScript library. This means that it's a collection of reusable JavaScript code that accomplishes common tasks. Since web developers often find themselves solving the same problems over and over again, it makes sense to collect useful bits of code into a single package that can be included and used in any project. The creators of jQuery have written code to smoothly and easily handle the most common and most tedious tasks we want to accomplish with JavaScript, and they've ironed out all the little differences that need to be worked out to get the code working in different browsers.
It's important to remember that jQuery is JavaScript, not a language of its own. It has all the same rules and is written the same way as JavaScript. Don't let this frighten you away—jQuery really does make writing JavaScript much easier.
jQuery's official tagline is "write less, do more." This is an excellent and accurate description of the jQuery library—you can really accomplish amazing things in just a few lines of code. My own unofficial tagline for jQuery is "find stuff and do stuff to it", because finding and manipulating different parts of an HTML document is extremely tedious with raw JavaScript and requires lines and lines of code, while jQuery makes that same task painless and quick. Thanks to jQuery, you can not only quickly create a drop-down menu but you can also create one that's animated and works smoothly in many different browsers.
