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