Frequently when building solutions using SharePoint you need to make use of reference data. This will be presented as look up type columns in the main list for your solution.
Internal data that is data stored within SharePoint can be stored within existing and new lists.
The user doesn’t need to see these lists therefore it makes sense to keep their user interface as uncluttered as possible. Leaving the list out of the navigation is one thing that can be done then it keeps the quick launch as small and useful as possible. Hiding the list will keep it out of the view all site content keeping this uncluttered as well. To hide a list open it in SharePoint designer and set the Hidden attribute
It is also a good idea to arrange a specific location for such look up type lists. Placing them in a high level site of the site collection allow them to be easily used from all child sites.
I described this here with SharePoint 2007 but it still applies equally to SharePoint 2010.
Simply define your look up column as a site column in the root site and to a list in the root site and you will be able to use it in any child site. The list can be maintained in one place.
Frequently however look up data already exists in external systems that can be accessed by web services or directly from a database.
SharePoint 2010’s business connectivity services can be used to hook up such data into the SharePoint environment. Depending on your particular needs this can be achieved with SharePoint Designer or for more advanced scenarios by using Visual Studio
Aspects of Business Connectivity Services are covered in many of our SharePoint 2010 Courses including