In summary, the hierarchy of storage in Oracle is as follows:
* A database is made up of one or more tablespaces.
* A tablespace is made up of one or more data files. These files might be cooked files in a file system, raw partitions, ASM managed database files, or a file on a clustered file system. A tablespace contains segments.
* A segment (TABLE, INDEX, and so on) is made up of one or more extents. A segment exists in a tablespace, but may have data in many data files within that tablespace.
* An extent is a logically contiguous set of blocks on disk. An extent is in a single tablespace and, furthermore, is always in a single file within that tablespace.
* A block is the smallest unit of allocation in the database. A block is the smallest unit of I/O used by a database.
References used : Oracle® Database Concepts 10g Release 2 (10.2)