If you are using Contribution Folders (folders_g), you want to use COLLECTION_GET_CONTENTS or a similar service.
As Jonathan mentioned you have the option to use following services as per your requirement :
COLLECTION_DISPLAY - http://docs.oracle.com/cd/E21764_01/doc.1111/e11011/c08_folders.htm#i1082379
If using Framework Folders then the following service :
Hope this helps .