The retrieval interface of the DBMS_METADATA API lets you specify the kind of object to be retrieved. This can be either a particular object type (such as a table, index, or procedure) or a heterogeneous collection of object types that form a logical unit (such as a database export or schema export). By default, metadata that you fetch is returned in an XML document.
To access objects that are not in your own schema you must have the SELECT_CATALOG_ROLE role. However, roles are disabled within many PL/SQL objects (stored procedures, functions, definer's rights APIs). Therefore, if you are writing a PL/SQL program that will access objects in another schema (or, in general, any objects for which you need the SELECT_CATALOG_ROLE role), then you must put the code in an invoker's rights API.
You can use the programmatic interface for casual browsing, or you can use it to develop applications. You would use the browsing interface if you simply wanted to make ad hoc queries of the system metadata. You would use the programmatic interface when you want to extract dictionary metadata as part of an application. In such cases, the procedures provided by the DBMS_METADATA API can be used in place of SQL scripts and customized code that you may be currently using to do the same thing.