What is the difference between rowid and rownum

ROWID is a pseudo column and is the unique address of a row.

Select rowid From dual Where user='SYSTEM';

Select user From dual Where rowid='AAAAB0AABAAAAOhAAA';

ROWNUM represents the number of the row after it has been retrieved from the the database after a query has been made.

Select rownum, name, email
From clients  
Where rownum > 500 
Order By name;