The largest logical structure in the database. A logical data storage space that maps directly to one or more data files. Tables, indexes, and other objects are created within a tablespce. A datafile can only be associated with one tablespace.
Segment is a set of extents that make up a schema object within a tablespace. Segments have several different uses, including being the storage holders for a schema object within a table space. Each segment belongs to one schema object. For example, a table has one segment containing all of its extents.
A schema object contains a wide variety of objects that can be created by users. Tables and indexes are probably the most common types of schema objects. Each schema object must be contained within one tablespace, with the exception of partitioned tables and partitioned indexes.