The finest level of granularity of the data base are the data blocks.
Data Block :: One Data Block correspond to SPECIFIC number of physical database space.
Extent :: Extent is the number of specific number of contigious data blocks.
Segments :: SET of Extents allocated for Extents. There are three types of Segments
a) Data SEGMENT :: Non CLUSTERED Table has data segment data of every table is stored in cluster data segment.
b) Index Segment :: Each Index has index segment that STORES data.
c) Roll Back Segment :: Temporarily store 'undo' information

The finest level of granularity of the data base are the data blocks.
Data Block :: One Data Block correspond to specific number of physical database space.
Extent :: Extent is the number of specific number of contigious data blocks.
Segments :: Set of Extents allocated for Extents. There are three types of Segments
a) Data Segment :: Non Clustered Table has data segment data of every table is stored in cluster data segment.
b) Index Segment :: Each Index has index segment that stores data.
c) Roll Back Segment :: Temporarily store 'undo' information