JavaScript is a client-side scripting language that is commonly used to create dynamic and interactive effects on web pages. It resides in the browser and is used to enhance user interfaces and experiences on websites. JavaScript code is made up of statements that are written within script tags. Functions are blocks of code in JavaScript that perform specific tasks, and variables are used to store and retrieve values. jQuery is a JavaScript library that simplifies client-side scripting by using CSS selectors and providing methods to easily manipulate HTML elements and run animations. Both JavaScript and jQuery code is typically linked from the HTML head or an external .js file and placed above other scripts.