A script is a small piece of program that can add interactivity to website. JavaScript runs on the client side of browser.
JavaScript is a lightweight, interpreted, object-oriented language with first-class functions, and is best known as the scripting language for Web pages, but it's used in many non-browser environments as well. It is a prototype-based, multi-paradigm scripting language that is dynamic, and supports object-oriented, imperative, and functional programming styles.
Developers can write various small functions, called event handlers using any of the scripting language and can trigger those functions using HTML attributes.