jQuery is a fast, lightweight, and feature-rich JavaScript library designed to simplify HTML document manipulation, event handling, animation, and AJAX interactions. It provides an easy-to-use API that abstracts the complexities of JavaScript and allows developers to write concise and efficient code.

Here are the key features and functionalities of jQuery:

1. DOM Manipulation: jQuery simplifies DOM manipulation by providing a set of methods and selectors that make it easier to traverse and manipulate the HTML document. Developers can easily select elements, modify their attributes and content, create new elements, and apply CSS styles.

2. Event Handling: jQuery provides an intuitive syntax for handling events such as clicks, keypresses, mouse movements, and more. Developers can attach event handlers to elements, bind multiple events simultaneously, and handle event delegation to efficiently manage events on dynamically added elements.

3. AJAX Support: jQuery includes a set of functions and methods to simplify AJAX interactions. It provides methods like `$.ajax()` and `$.get()` to make HTTP requests, retrieve data from the server, and update the page content without a full page reload. jQuery abstracts the complexity of the underlying XMLHttpRequest object and provides a consistent API for AJAX operations.

4. Effects and Animations: jQuery offers built-in animation and effects methods that allow developers to add visual effects, transitions, and animations to web pages. These methods make it simple to fade elements in or out, slide elements, animate CSS properties, and create custom animations.

5. Utility Functions: jQuery provides a wide range of utility functions that simplify common tasks, such as manipulating arrays and objects, handling asynchronous operations, working with browser and device information, and more. These utility functions help streamline the development process and improve code readability.

6. Cross-Browser Compatibility: jQuery handles cross-browser inconsistencies and variations, providing a unified interface for JavaScript operations across different browsers. It abstracts the complexities of browser-specific implementations and ensures consistent behavior across platforms.

7. Plugin Architecture: jQuery has a vast ecosystem of plugins developed by the community. These plugins extend the core functionality of jQuery and provide additional features, such as sliders, date pickers, image galleries, form validation, and more. Developers can easily incorporate these plugins into their projects to enhance functionality and save development time.

jQuery was widely adopted in web development for many years, and it played a significant role in simplifying JavaScript development. However, with advancements in modern JavaScript and the native capabilities of browsers, the usage of jQuery has declined. Nevertheless, it remains relevant for maintaining legacy projects and working with older systems that rely on jQuery.