There are three distinct layers of content that are used when developing a web page. These layers allow different web designers, who may have different skill sets to work on different parts of a web page.
The content layer is the text, images, videos, etc that are included in the web page. You can think of the content layer as containing the information held on the web page.
The presentation layer is the way the website will be displayed in a web browser . This is usually defined by stylesheets (CSS).
The behaviour layer allows interaction with the web page and is usually defined by the programming included in many web pages. For example when completing an online form, a user will be able to click a submit button, access drop down lists, etc. The input from the user is then usually validated in some way before it is submitted. These are examples of actions that would be part of the behaviour layer.