A content type is a blueprint of how content items of that type look like: it defines the set of content parts that make up the content types.
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 is part of the Orchard Dojo Library!
This page and many others here are part of the open-source, portable package of Orchard 1.x goodies: the Orchard Dojo Library for Orchard 1.x. 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!
Note that the Library also has an Orchard Core version, check it out here!