jQuery, at its basic level, is a DOM manipulation library. The DOM is the representation of all the components of a web-page in the tree structure.
jQuery untangles the syntax for selecting, finding & manipulating these DOM components. Taking an example – one can make the use of jQuery to locate an element within the document with a defined property, altering one or more of its attributes, and causing it respond to an event. The modular approach with jQuery enables the building of powerful dynamic web-pages & applications.