jQuery is a free, open-source JavaScript library that simplifies HTML document traversal and manipulation, events, animations, and Ajax interactions for rapid web development. It provides abstractions for selecting elements, handling events, performing animations, and developing Ajax applications. jQuery can be included in a web page by linking to a local copy or a public CDN, and has two usage styles via the $ function or $.prefixed utility functions. Plugins allow extending jQuery's functionality in many areas like Ajax, web services, and more.