This document provides an overview of jQuery, an open-source JavaScript library. It defines jQuery as not being a language itself, but rather well-written JavaScript code that makes it easier to use JavaScript on websites. Some key features of jQuery discussed include selecting and manipulating HTML/CSS, JavaScript effects and animations, AJAX capabilities, and events/effects like fade in/out and slide up/down. The document also reviews how to download, install, and use jQuery in a basic example.