JavaScript is a dynamically-typed scripting language that is used to develop client-side (and now server-side) logic. It was designed with web designers in-mind, allowing them to create dynamic content on webpages with little programming knowledge or experience.
Invented in the 90's, the language quickly became popular for both designers and malicious users. Many security exploits exist today because of it, including XSS. JavaScript is maintained by the ECMAScript International Board.