One of the new features of Oracle 12c is multitenant databases. Multiple databases can be plugged in a big container database.
Container database (CDB) is the primary database that comprises 0, 1 or many pluggable databases (PDB). This holds the controlfiles, datafiles, undo, tempfiles, redo logs and also the data dictionary for those objects that are owned by the root container. It is also called CDB$ROOT
Pluggable Database (PDB) – these are the databases stored within the primary container database. Each PDB is a set of schemas, objects and non-schema objects. PDB is made up of only datafiles and tempfiles to handle its own objects which includes its own data dictionary containing information about only those objects specific to the PDB. PDBs appear to the end users as a non-CDB database.
Seed PDB - A default PDB that the system uses as a template to provision other user-created PDBs. It is called PDB$SEED