Dense Rank is used for assigning a precedence or rank to 
each row in the result set relative to other rows in the 
result set.

The DENSE_RANK function computes the rank of a row in an 
ordered group of rows.Rows with equal values for the 
ranking criteria receive the same rank.

For eg.

To Show the set of sales people who make the top 3 
salaries - that is, find the set of distinct salary 
amounts, sort them, take the largest three, and give me 
everyone who makes one of those values.

SELECT * FROM (
  SELECT deptno, ename, sal,
         DENSE_RANK()
  OVER (
    PARTITION BY deptno ORDER BY sal desc
  ) TopN FROM emp
)
WHERE TopN <= 3
ORDER BY deptno, sal DESC
/

    DEPTNO ENAME             SAL       TOPN
---------- ---------- ---------- ----------
        10 KING             5000          1
           CLARK            2450          2
           MILLER           1300          3

        20 SCOTT            3000          1  <--- ! (in 
                                            case of rank
                                            JONES will have
                                            3 and dense 
                                           rank will have 2)
           FORD             3000          1  <--- !
           JONES            2975          2
           ADAMS            1100          3

        30 BLAKE            2850          1
           ALLEN            1600          2
        30 TURNER           1500          3