A content type only consists of content parts, even if it seems that you can directly attach content fields to a type from the admin UI. When you attach fields from the admin UI in reality an invisible part is created that has the same name as the content type (e.g. a Page part is created for the Page content type) and the fields get attached to that.
Examples of some basic content types: Page or Blog Post (both containing e.g. Title Part and Body Part).
This page and many others here are part of the open source, portable package of Orchard goodies: the Orchard Dojo Library. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!