JavaScript:  

JavaScript is what is called a Client-side Scripting Language. That means that it is a computer programming language that runs inside an Internet browser (a browser is also known as a Web client because it connects to a Web server to download pages).

The way JavaScript works is interesting. Inside a normal Web page you place some JavaScript code. When the browser loads the page, the browser has a built-in interpreter that reads the JavaScript code it finds in the page and runs it.