Within Microsoft Dynamics CRM 4.0 you have as an adminstrator of the system and ability to create custom screens, with your own custom attributes or fields. Microsoft Dynamics CRM 4.0 will automatically take care of creating the necessary structure to link your newly created entity to the back end CRM database.
However you do not need to stay within the boundries of Microsoft Dynamics CRM 4.0 user interface creation, you may wish to retrieve data from an external data source and display it in your own customised screens, Microsoft Dynamics CRM 4.0 architecture allows this too.