In Oracle ,everything is stored in blocks.An extent is a set of contignous blocks allocated for a database object.A segment is a collection of all the extents allocated for a database object.
So when u create any object in Oracle such as a table,index,rollback segment.For each of these a segment is created.
When u create a table,a data segment will be created to store the table data.When u create a index,an index segment will be created to store the index values.When u create a rollback segment ,a rollback segment will be created to store the undo data.
The segment name is the same as the table name or index name.So when u create a table named emp ,an emp segment gets created to store the data of emp table,U can verify it through dba_segments table.
So a segment will contain all the extents which are allocated for a database object.This segments get created through ur storage paremeters of ur table or index or rollback segment
If u have more doubts in this please mail me to email@example.com