Umbraco DocTypes are versatile structures of information. They can be thought of as objects with properties, and can be inherited.
Properties of the DocType are typically set when the instance of a document is created. Content of a web-page, for example, is added to the page (an instance of a particular DocType) by the umbraco user when they create a new page.
DocType properties can be referenced in transforms, macros, templates, and rich-text content.
DocTypes are unique, in the sense used here, to umbraco, along with umbraco Macros. In contrast, the XSLT, CSS, and Template files use well established standards. I think DocTypes are stored in the database--I haven't spotted any files directly related to them. Maybe in umbraco.config?
You can Export a DocType through its context menu. You can import a DocType in the DocType context menu in the Settings section. This export and import process works well, and is pretty smooth.
