Oracle Database Physical Architecture

Tablespaces are logical structural overlays containing one or more datafiles. A
schema (owner or Oracle user) is also a logical overlay structure. A schema can be spread across multiple table spaces and contains segments or  Database objects such as rollback segments,temporary segments or tables, and indexes.Segments are physical or logical subdivisions within schemas and can be accessed across different schemas. Segments are physically broken up into extents. An extent is a number of blocks added to a segment (datafile) when an object requires more space.