DESC Keyword

The default sort order is ascending when using the ORDER BY clause , you can reverse the order in which rows are displayed by using the DESC keywords. It placed after column name in the ORDER BY clause
For Example 
SELECT EMPNO,ENAME
FROM EMP
ORDER BY SAL DESC;
EMPNO ENAME             SAL

----- ---------- ----------

 7839 KING             5000

 7788 SCOTT            3000

 7902 FORD             3000

 7566 JONES            2975

 7698 BLAKE            2850

 7782 CLARK            2450

 7499 ALLEN            1600

 7844 TURNER           1500

 7934 MILLER           1300

 7521 WARD             1250

 7654 MARTIN           1250

EMPNO ENAME             SAL

----- ---------- ----------

 7876 ADAMS            1100

 7900 JAMES             950

 7369 SMITH             800

Select ename,sal,mgr

From emp

Order by mgr DESC;

ENAME             SAL        MGR

---------- ---------- ----------

KING             5000

SMITH             800       7902

JONES            2975       7839

CLARK            2450       7839

BLAKE            2850       7839

ADAMS            1100       7788

MILLER           1300       7782

ALLEN            1600       7698

MARTIN           1250       7698

WARD             1250       7698

JAMES             950       7698

ENAME             SAL        MGR

---------- ---------- ----------

TURNER           1500       7698

SCOTT              3000       7566

FORD               3000       7566