How do I create a CT within a CT as shown in the image?
To complete James's answer, the lookup is probably used for "linking" the list.
In order to get such display, the easiest and quickest way is to use SPD to create a custom dispform that contains an aggregated data source and a XsltViewWebPArt (I said easy, but it'sz actually a source of headache to understand how this works).
Another way is to create a custome template in 14Hive\controltemplates, and set the parent content type to use this template. Within the template, you can add the "child" view.
A last way (but I don't think it's a good idea to use it), is ti create a custom field type, to implement the display like this.
You can actually add related lists views to list forms directly from the SharePoint UI.
1 - Open a List (e.g. Tasks.
2 - Select the List tab from the Ribbon
3 - In the Customize List Group, select "Default Display Form" from the Modify Form Web Parts menu.
4 - Highlight the main form control (the area around the web part zones)
5 - From the Page Tools > Insert tab, use the Related Lists command from the Relationships group.
Pretty nice - you don't even need SharePoint Designer anymore to add relationship views to list forms...
Looks like there's a lookup to a task list in your image. The Task List has a column of type "Lookup" which connects to the Reservations list, so one reservation can have many tasks.