Segment is the entity that uses the physical storage space within the database. Some of the common segment types are:

• Table: Stores data in columns and rows.
• Index: Improves the access to the table data. Helps in increasing the performance of data extraction.
• Rollback: Rolls back the uncommitted data.
• Partition: Divides a table into smaller pieces for performance.

As shown in the figure, group of extents make a segment. Each extent is made of a collection of oracle database blocks , Minimum size of extent is five blocks. The database block sizes are in multiples of 2 like 2k,4k,8k and 16k.