When we speak about content types I think there are some common general principles:

• all the content types are children of  the visible Item content type (there is also a Parent for this, called _System but is hidden).
• you should look at the content types as they are Business Entities (with the columns being the fields). By design they have a parent and they inherit all the columns defined within the parent.
• a great advantage provided by the content types is that you have the possibility to enforce the data structure throughout the site collection. For example if you want to add a new column which must be pushed throughout all the Page Layout Content Type, you could add it in the Page content type.
• you can define documents templates and page layouts and attach them to the content types. Every time when you are going to use that content type in a library the templates will be available too.

You can find detailed information on the Content type and workflow planning (SharePoint Server 2010) article on TechNet.
In the image below you can see how the publishing feature page content types are defined as children of the system content types (each child content type shows only the incremental differences from the parent):