jQuery is a multi-browser JavaScript library designed to simplify the client-side scripting of HTML. It was released in January 2006 at BarCamp NYC by John Resig. It is currently developed by a team of developers led by Dave Methvin. Used by over 65% of the 10,000 most visited websites, jQuery is the most popular JavaScript library in use today.

jQuery is free, open source software, licensed under the MIT License. jQuery's syntax is designed to make it easier to navigate a document, select DOM elements, create animations, handle events, and develop Ajax applications. jQuery also provides capabilities for developers to create plug-ins on top of the JavaScript library. This enables developers to create abstractions for low-level interaction and animation, advanced effects and high-level, theme-able widgets. The modular approach to the jQuery library allows the creation of powerful dynamic web pages and web applications.

Microsoft and Nokia have announced plans to bundle jQuery on their platforms. Microsoft is adopting it initially within Visual Studio[9] for use within Microsoft's ASP.NET AJAX framework and ASP.NET MVC Framework while Nokia has integrated it into their Web Run-Time widget development platform. jQuery has also been used in MediaWiki since version 1.16.

Features 

DOM element selections using the multi-browser open source selector engine Sizzle, a spin-off of the jQuery project
DOM traversal and modification (including support for CSS 1–3)
DOM manipulation based on CSS selectors that uses node elements name and node elements attributes (id and class) as criteria to build selectors
Events
Effects and animations
AJAX
Extensibility through plug-ins
Utilities - such as user agent information, feature detection
Compatibility methods that are natively available in modern browsers but need fall backs for older ones - For example the inArray() and each() functions.
Multi-browser (not to be confused with cross-browser) support.