Primary Key : Concept

A relational database is based on the concept that the user that
queries the database needs not remember the physical location of
the record that he/she needs access to.

the user only needs to remember the table name and
the value that the user associates to.

that's why a relational database must contain a unique identifier
for each record that exists in the relational database.

thus the concept of primary key arises that provides a unique
identifier for each record called as the "PRIMARY KEY".

ORACLE Logical Database Structures

Tablespaces

A database is divided into logical storage units called
tablespaces,which group related logical structures together.
One or more datafiles are explicitly created for each tablespace
to physically store the data of all logical structures in a
tablespace.

Oracle Data Blocks

Oracle database data is stored in data blocks. One
data block corresponds to a specific number of bytes of
physical database space on disk.
The standard block size is specified by the DB_BLOCK_SIZE
initialization parameter.

Extents

An extent is a specific number
of contiguous data blocks, obtained in a single allocation,
used to store a specific type of information.

Segments

A segment is a set of extents allocated for a certain logical
structure.

Types Of Segments -

Data segment
Index segment
Temporary segment
Rollback segment