An Oracle Database can contain a portable collection of schemas, schema objects, and nonschema objects that appear to an Oracle Net client as a separate database. This self-contained collection is called a PDB. A CDB can include zero, one, or more PDBs. Oracle Database 12c allows you to create many PDBs within a single CDB. Applications that connect to databases view PDBs and earlier versions of Oracle Database in the same manner.
EM Express enables database administrators to manage a CDB and its PDBs.
See Also:
Oracle Database Concepts and Oracle Database Administrator's Guide for detailed conceptual information about CDBs and PDBs