JavaScript is a programming language that helps make a web page interactive.
When you view a slideshow or get annoying popup ads - that's JavaScript. If you see your Facebook Timeline update or Google autocomplete a search term - that's JavaScript.
When most anything changes on a page without you clicking a link - that's JavaScript!
JavaScript is one of the few programming languages that can work in your browser. Because of its ubiquity and how easy it was to learn, for many years it was dismissed by professionals as being a toy for amateurs.
This all changed around ten years ago with the advent of "AJAX" and "Web 2.0." These were sets of technology that used JavaScript in a whole new way, allowing the Web to be way more interactive than it had been before. So next time you use a Google Spreadsheet or watch a Tweet update on a web page, you can know what's going on - that's JavaScript!
Cocktail Party Fact
You might have heard of 'Java,' but that's actually a completely different language. It came out around the same time as JavaScript, and they only share a name thanks to a marketing scheme! The joke goes "Java is to Javascript what Car is to Carpet."
JavaScript's fancy real name is ECMAScript, but you should only call it that when you’re angry that it hasn't done its dishes.