jQuery

jQuery is a fast and concise JavaScript library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. In simpler terms, jQuery is a collection of JS code that you can use in websites.

jQuery allows easy finding and changing of elements in the HTML document. It also makes it easier to handle user actions like clicks, mouse movements, key presses and so on.

jQuery also includes built-in effects and animations, allowing developers to add visual interest to the web page. It also simplifies the process of sending and receiving data asynchronously without interfering with the display and behaviour of the existing page.

Finally, jQuery can handle a lot of the headaches in cross-browser capability. It smooths over the differences in how different browsers respond to JavaScript code, so you do not have to write different code for different browsers.