Why do we need three languages HTML, CSS and JavaScript to run some great codes?

Question

While going through some awesome codes I saw the code is divided into mainly three languages HTML, CSS, JavaScript ... Does the whole code gets divided into three parts for simplicity or the same code is written in different languages? And How can I use this technique in my code?

Answer 1

Remember

HTML .. structure
CSS .. presentation
JS .. behaviour

keeping that in mind
produces fantastic effects

Answer 2

HTML shows the content layout,
CSS adds beauty to the layout,
JS makes content dynamic. (tells what is to be done with content).

//I stand to be corrected