Dependency may not be the right word here - but basically my question is regarding what happens to the data inside a sharepoint content database if that data was created, for example, by using a custom content type - but I then remove that content type.
You cannot remove Content Type as long as Data Still relies on it. SharePoint wont' allow you. E.g. Even after deleting the Item (or the whole list for that matter), unless you empty the Recycle-Bin you won't be able to Delete the content type.
Basically SharePoint uses a Dependencychecker to ensure you are not doing it. If you delete it by hand from Database you risk big.
Drop a comment if you need deeper explanation C:\Marius