Oracle storage management is built on four basic entities: data blocks, extents, segments, and tablespaces.
These entities have a hierarchical relationship -- extents contain data blocks, segments are made up of extents, and tablespaces are made up of segments.
These relationships are shown in the MouseOver below.

The tablespace, the highest level of physical organization in an Oracle database, is the interface between the physical storage of the database and the logical structures within the database. The tables, indexes, and other objects within a database are assigned to a tablespace. The tablespace is also the interface to the physical files managed by the underlying operating system. Each tablespace is associated with one or more physical files.
A tablespace is generally the smallest unit of physical storage that can be addressed by standard Oracle operations, such as backup and recovery.
This does not mean that you can ignore the underlying levels of space management. Because each of these levels of physical organization is dependent on the other, the characteristics of each level of storage affect the higher levels.
The next lesson is about data blocks.