Hi raji,
This is a basic question and I am sure if you search on internet you can find lot's of information.
Job is general description for a position but a position has one to one relationship with EEs in a organization. Imagine you have 3 position(employee) in a organization and they are time management expert, payroll expert and recruitment expert. These are positions of them but their job can be "HR expert".
By a top down design from job to position you can make sure will generalaize requirements of positions into job and this will make maintenance of system simple for future.
Regards,
Omid