A PDB is a user created set of schemas, objects, and related structures that appears logically to a client application as a separate database.
Every PDB is owned by SYS, regardless of which user created the PDB. SYS is a common user in the CDB, which means that this user that has the same identity in the root and in every existing and future PDB within the CDB. PDBs can be plugged into to CDBs. A CDB can contain multiple PDBs. Each PDB appears on the network as a separate database.