Oracle Database Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 83

Figure 3-4: Logical architecture of Oracle database. Tablespace is like
small departmental level database. Logically database is composed of
tablespaces. Each table space can be visualized as a department level
database e.g for each department of a company you would be having
one tablespace. Each tablespace is composed of segments and Oracle
allots a segment each time you create a database object e.g. tables,
views etc using CREATE statement. Each Segment is composed of
Extents and extents are nothing but contiguous DB Blocks. DB Block
is usually the even multiple of OS Block and its value is specified in
parameter file (init.ora).