Very Simple question! But should be explained a lot.

Databse data is stored in data_file's of the databse. For every table's data some space from the data_files would be allocated. This space is known as data segment. Say, you have a table named EMP. There could be a data segment 'EMP_DATA_SEG' for this table. So, when you query the table using the the query 'SELECT ENO FROM EMP', Oracle server would parse the query ( check for syntax, table availability, your authentication on that table etc) and then would go to the data segment to get you the the data based on the condition that your query has specified.

Hope you would have got some idea.

Regards,

Maria Antony.