jQuery, at its core, is a DOM (Document Object Model) manipulation library. The DOM is a tree-structure representation of all the elements of a web-page and jQuery makes finding, selecting, and manipulating these DOM elements simple and convenient. For example, jQuery can be used for finding an element in the document with a certain property (e.g. all elements with an h1 tag), changing one or more of its attributes (e.g. color, visibility), or making it respond to an event (e.g. a mouse click).
jQuery provides a new paradigm for event handling in JavaScript beyond basic DOM element selection and manipulation. The event assignment and the event callback function definition are done in a single step in a single location in the code. jQuery also aims to incorporate other highly-used JavaScript functionalities (e.g. fade ins and fade outs when hiding elements, animations by manipulating CSS properties).
The advantages of using jQuery are: