The finest level of granularity of the database 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
