JavaScript manipulation of the DOM : Start creating Dynamic and Interactive Content.  

Learning how to interact with the DOM, will get you started quickly with more interaction on your webpages.

The Document Object Model is a model of your HTML document represent within JavaScript as objects.

Elements are within the HTML page rendered out by the browser.  JavaScript engine in the browser then create representations of the elements as JavaScript Objects. Which in turn gives you the developer a whole lot to work with, the sky’s the limit with what you can do at this point.  You can communicate with these objects using JavaScript and then manipulate them.  Any changes you make will be visible within the webpage itself.

Use JavaScript to select elements from your HTML code.   All the web page elements are represented within the DOM.  Document Object Model.  Learning to better utilize the DOM will allow you to create more advanced user interactions, and update the content dynamically.