SQL & PL/SQL

dear members,

I have a table named 'students' which has 12 rows in it. when i try to execute the query
select * from students
where rownum>10;

it did not give any rows in the output. I tried to give rowid=10 still no rows were retrieved in the output, but if i give rowid<10 then it fetched 9 rows in the output.

my questions are :

1. why rownum is behaving differently in all the 3 cases that i mentioned above?
2. what is the difference between ROWID and ROWNUM?

I am using sql * plus with oracle version 9i. I would appreciate your help.

cheers
sandeep