A content type consists of one or many fields. A field is defined by a field type. An example for a content type is blog post. An example for a field is text field. Its field type is text field and it has a couple of other settings (#maxlength, #description, etc.). Another example would be image upload field. The field type would be image.
Both, the text field and the image upload field are part of the content type blog post. A user can create one or many blog posts by entering text into the text field and upload an image. The #required setting of a field defines, if it is mandatory or optional.
A VR Theme can define one or many content types in its configuration file config.php.
A detailed list of all field types can be found here.