Temp Tablespace - used for SortOperations and for storing global temporary tables.
SQL operations that might require disk sorting are: CREATE INDEX, ANALYZE, Select DISTINCT, ORDER BY, GROUP BY, UNION, INTERSECT, MINUS, Sort-Merge joins, etc.
Note : dba_free_space does not record free space for temporary tablespaces.
Free Space :
SQL> select TABLESPACE_NAME, BYTES_USED, BYTES_FREE from V$TEMP_SPACE_HEADER;
TABLESPACE_NAME BYTES_USED BYTES_FREE
------------------------------ ---------- ----------
TEMP 52428800 52428800