Oracle Database 12.1 is the most latest version of Oracle Database.
There is in place upgrade available from versions lower than 12.1. Upgrade from lower version like 10g or 11g should be performed out of place.
Oracle 12C is the first in Oracle database version with multi-tenant architecture.
A CDB includes:
Root-> Named CDB$ROOT, stores Oracle-supplied metadata and common users.
The seed -> named PDB$SEED, is a template that you can use to create new PDBs.
A CDB has exactly one seed.
Common Users and Local Users in CDB/PDB:
A common user can log in to the root and any PDB in which it has privileges. The operations that a common user can perform depend on the privileges granted to the common user. Some administrative tasks, such as creating a PDB or unplugging a PDB, must be performed by a common user.
A local user is a user that exists in exactly one PDB. Database administrators connects to the CDB as common users, and they manage attributes of the CDB, the root as well as some attributes of PDBs. For example, database administrators can create, unplug, plug in, and drop PDBs.
Database administrators can also connect to a specific PDB as a local user and then perform a subset of management tasks on the PDB. The subset of tasks are those required for the PDB to support an application. For example, these can include management of tablespaces and schemas in a PDB, specification of storage parameters for that PDB, and changing the open mode of the current PDB.