What's the difference between RANK() and DENSE_RANK() functions in oracle?

What's the difference between RANK() and DENSE_RANK() functions? How to find out nth salary in the following emptbl table?

DEPTNO  EMPNAME    SAL

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

10       rrr    10000.00

11       nnn    20000.00

11       mmm    5000.00

12       kkk    30000.00

10       fff    40000.00

10       ddd    40000.00

10       bbb    50000.00

10       ccc    50000.00

If in the table data having nulls, what will happen if I want to find out the nth salary?