Now let us consider an example..

Employee -- Class
empno
emp_name
dept_id

Dept---Class
dept_id
dept_name
dept_location

so 1-to-Many relationship occurred in the above case.
Now i am stuck up in understanding the concept.
When is this mapping considered and go the following questions
1) Is this mapping considered only while fetching the data from database?
2) Is this mapping considered only while storing data to database?
3) Or this is considered for both 1) and 2)
4) Why mapping is used?
5) How to decide which mapping to be used for eg lets consider One-to-Many relationship {Unidirectional or Bidirectional}?

Can you explain a little more with the above example