The sort area is amount of memory a user gets for sorting purposes (typically 64 KB).
(this applies to DEDICATED_SERVER connection mode)
If the sort can not be done in that memory it will use the temporary tablespace for that user to do the sort.
The sort is needed in case of an order by or distinct or union clause in a query.
In SHARED_SERVER mode (MTS,connection sharing ) the sort_area for the user session is actually contained in the SGA.
This is mostly done with lots of users.
Search for DEDICATED SERVER and Multithreaded Server (MTS)
for the details.