Oracle 12c : PDB and CDB

Oracle Database 12c introduces a new multitenant architecture. It enables an Oracle database to contain a portable collection of schemas, schema objects, and nonschema objects that appears to an Oracle Net client as a separate database. This self-contained collection is called a pluggable database (PDB). A multitenant container database (CDB) is an Oracle database that includes zero, one, or many user-created PDBs

The screen shot below will give you a good idea about the  CDB and PDB in oracle 12c

-click on image to enlarge-

[content given below is just copied from Oracle document]

A container is either a PDB or the root container (also called the root). The root is a collection of schemas, schema objects, and nonschema objects to which all PDBs belong.
Every CDB has the following containers:
Exactly one root
The root stores Oracle-supplied metadata and common users. An example of metadata is the source code for Oracle-supplied PL/SQL packages . A common user is a database user known in every container  The root container is named CDB$ROOT.

Exactly one seed PDB
The seed PDB is a system-supplied template that the CDB can use to create new PDBs. The seed PDB is named PDB$SEED. You cannot add or modify objects in PDB$SEED.

Zero or more user-created PDBs
A PDB is a user-created entity that contains the data and code required for a specific set of features. For example, a PDB can support a specific application, such as a human resources or sales application. No PDBs exist at creation of the CDB. You add PDBs based on your business requirements.

Ref:
http://docs.oracle.com/cd/E16655_01/server.121/e17633/cdbovrvw.htm