Basic Presentation Explanation
Below is the wireframe for the "Basic" layout that the Launch Sitecore site uses.
This simple Layout (a term Sitecore uses for the .aspx Web Form itself) provides four separate regions. These regions are identified by Sitecore Placeholders. If you look at the actual .aspx file (which is in your project at /layouts/LaunchSitecore/Basic/BasicLayout.aspx), you'll see the Sitecore controls called out in the source. For instance:
<sc:Placeholder ID="phHeader" Key="header" runat="server" />
This tag identifies the Header Placeholder region in the wireframe above. Sitecore will replace this placeholder with any controls that have been identified in the Item's presentation details for that Placeholder.
To see an Item's presentation detail, use the Content Editor and choose Presentation-->Details. You will see a dialog similar to the screenshot below:
When you click Edit, you will see the specific Placeholders a control is destined for.
The Show Fields control (which happens to be a Sublayout, or .ascx control) is set up to be inserted where the "content" placeholder resides in our Layout file.