These are the predefined packages,created at the time of oracle instalation.These are user for various purposes.
To get the list
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SELECT OBJECT_NAME FROM dba_objects where OBJECT_TYPE='PACKAGE' ORDER BY OBJECT_NAME
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You will get around 850 predefined packages.
To get the list of procedures and functions of the package
describe the package like u do in case of a table.
The most frequently used package that u might have came across is DBMS_OUTPUT
It is not practically possible to explain all the proc/func of these as the number is too vast.