The layout concept of Snap! Websites is what forms a page. You select one layout and then assign content to display in the different boxes defined in that layout.
The content being added to the layout may be dynamic content and it may be conditional. For example, it could be the list of the last 5 posts on your website when an anonymous user comes to visit your site. However, the same box could instead show the most important 5 points you need to put your attention on (i.e. posts that need to be moderated, to do items that become due, new emails to check out, etc.)
In that case we have PDFs (Page Dynamic Fields.)
The system should define a large set of layouts and later versions should offer the user a way to create new layouts.